Zend PHP Certification Study Guide- P10


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Zend PHP Certification Study Guide- P10

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  1. 164 Chapter 10 Stream and Network Programming n whether the stream connection has timed out or not n whether the stream has blocked or not n whether all data has been read from the stream or not To get stream metadata, use the stream_get_meta_data() function. Here is the output from running the code example: array(6) { [“wrapper_type”]=> string(9) “plainfile” [“stream_type”]=> string(5) “STDIO” [“unread_bytes”]=> int(0) [“timed_out”]=> bool(false) [“blocked”]=> bool(true) [“eof”]=> bool(false) }
  2. Introducing Streams 165 echo “Metadata from a connection to: http://www.php.net/\n\n”; $fp = fopen(“http://www.php.net/”, “r”); stream_filter_append($fp, “string.rot13”); var_dump(stream_get_meta_data($fp)); fclose($fp); ?> Pipelines Data in a stream flows along one of two pipelines: nData sent down a stream from your PHP script to the destination file or network server flows down the write pipeline. nData retrieved from the file or network server flows up the read pipeline. Some streams will have both pipelines, but some streams will only have a read pipeline or a write pipeline. What Is the Stream Transport? At the far end of the pipeline, the furthest away from your PHP script, is the stream transport.The stream transport is a piece of code that enables the file wrapper to talk directly with whatever the stream is connected to. PHP comes with a number of built-in transports: n STDIO The STDIO transport is used to talk to normal files, special resources such as stdin and stdout, and any other types of file supported by your underlying operating system. n socket The socket transport is used to talk to (possibly remote) servers over the network. PHP automatically chooses the correct transport to use with your choice of file wrapper. What Is the Stream Context? The stream context is a piece of data about the stream and about the data passing along the stream. It is used to pass additional options to the file wrapper or stream transport. You create the context using the stream_context_create() function, and then pass it as a parameter to fopen() or fsockopen(). Different file wrappers and stream transports accept different options.You can pass options to both the file wrapper and underlying stream transport at the same time.
  3. 166 Chapter 10 Stream and Network Programming How Do Streams Affect Me? Most of the time, you will be using streams via fopen() and the file wrappers. PHP always manages the stream for you, and you can pay it little mind under these circum- stances. If you have to directly interact with the stream, it will probably be to pass options through to the file wrapper via a stream context, or to retrieve extra information from the file wrapper via the stream’s metadata. The other time that you will need to work more closely with the stream is if you are writing PHP code to talk over the network to remote servers and services using net- work protocols. Connecting to Remote Hosts Using Sockets When you access a normal file, all file operations ultimately are handled by your com- puter’s operating system.The operating system creates a resource called a file handle. File handles make it easy for the operating system to understand which file PHP is reading from or writing to. When you access a (possibly remote) server over the network, all the operations on this connection are also handled by your computer’s operating system. Instead of creating a file handle, the operating system creates a resource called a socket. File handles and sockets are very similar, and through the PHP Streams architecture, PHP tries to keep the differences to a minimum. When Should I Use a Socket Instead of a File Wrapper? Some file wrappers allow you to access (possibly remote) network servers. For example, the http file wrapper allows you to retrieve pages from a web server. Unlike sockets, file wrappers will hide the details of supporting the application-layer network protocol. So why would you want to use a socket instead? You must use a socket if you want to connect to a (possibly remote) network server that there is no file wrapper for. An example would be connecting to the memcached caching server.There is no file wrapper that supports the memcached network protocol. You must use a socket if you want to do something that the file wrapper cannot do— but is possible through the underlying network protocol. An example would be sending an XML-RPC message to a (possibly remote) web server. XML-RPC involves sending XML messages to and from the web server, using the HTTP network protocol.The http file wrapper only supports reading from a web server; it does allow you to write data to the web server. But the underlying HTTP network protocol does support writ- ing data to a web server, and you can access this network protocol by using a socket rather than by using a file wrapper.
  4. Connecting to Remote Hosts Using Sockets 167 What Network Transports Does PHP Support? You can find this information in the “List of Supported Socket Transports” appendix in the PHP Manual. n tcp This transport allows you to connect to (possibly remote) network servers using the connection-orientated Transmission Control Protocol—the TCP part of TCP/IP. n udp This transport allows you to connect to (possibly remote) network servers using the connection-less User Datagram Protocol—part of the TCP/IP network protocol. n ssl This transport allows you to connect to (possibly remote) network servers using Secure Sockets Layer encryption. SSL runs over TCP connections. n tls This transport allows you to connect to (possibly remote) network servers using Transport Layer Security encryption.TLS runs over TCP connections. n unix This transport allows you to connect to services running on the local computer using the connection-orientated UNIX Domain protocol. n udg This transport allows you to connect to services running on the local computer using the connection-less UNIX Domain protocol. How Do I Open a Socket? You can create a socket using the fsockopen() and pfsockopen() functions.You tell PHP what type of network transport you want to use by prefixing the transport to the name or IP address of the server you want to connect to.
  5. 168 Chapter 10 Stream and Network Programming // note that what comes back is a redirect, and not the front-page itself // this is an example of one of the many things that the http file wrapper // automatically (and transparently) handles for us $fp = fsockopen (“tcp://www.php.net”, 80, $sock_errno, $sock_errmsg); fwrite ($fp, “GET /\n”); while (!feof($fp)) { echo fgets($fp) . “\n”; } fclose($fp); ?> Sockets created using fsockopen() are automatically closed by PHP when your script ends. Sockets created using pfsockopen() are persistent. Persistent Sockets Sockets created using pfsockopen() remain open after your script has finished.When your next script calls pfsockopen() with the same hostname and port, PHP will reuse the socket that you opened last time—provided that the socket is still open. PHP only persists sockets inside a single process. nIf you are using a CGI-BIN version of PHP, the next time your script runs, the old PHP process will have terminated.Your persistent socket will have been closed automatically by PHP when your script finished running. nIf you are using mod_php, or a FastCGI version of PHP (such as Zend’s WinEnabler under IIS), there is a pool of reusable PHP engines.When your script runs, it might run inside the same copy of the engine as last time—or it might not. If your script runs inside a different copy of the engine, the call to pfopensock() will open up a new socket connection. Remote servers (and especially by any firewalls in between) will automatically close per- sistent sockets if the socket isn’t used for a period of time. Timeouts When Opening a Socket If you don’t provide the timeout parameter to fsockopen(), PHP uses the value of default_socket_timeout from the php.ini settings. The timeout parameter to fsockopen(), and the default_socket_timeout setting, only affect attempts to open the socket.This timeout is not used at all for read and write operations.
  6. Connecting to Remote Hosts Using Sockets 169 How Do I Use a Socket? The PHP Streams architecture allows you to treat socket connections as you would another type of stream.To read from a socket, you can use fread() and others.To write to a socket, you can use fwrite() and others. fread() and fwrite() are binary safe—you can use them to read and write any type of data that you need to. Blocking Mode By default, when PHP creates a new socket, it switches on blocking mode for that stream. When blocking mode is on, any functions that attempt to read data from the stream will wait until there is some data available to be read—or until the socket is closed by the remote server. You can switch blocking mode off by using stream_set_blocking():
  7. 170 Chapter 10 Stream and Network Programming // once again, we will make a connection to the PHP Project’s webserver, // and attempt to read from the socket without having told the webserver // what page we want it to serve // // the difference this time is that we will switch off blocking mode first // // finally, we will dump the return value from fgets(), so you can see // what fgets() returns when trying to read from a blocked stream $fp = fsockopen(“tcp://www.php.net”, 80, $sock_errno, $sock_errmsg); stream_set_blocking($fp, false); echo “Attempting to read from the stream ... this will fail and return\n”; echo “immediately\n\n”; $result = fgets($fp); fclose($fp); echo “fgets() has returned:\n”; var_dump($result); ?> Read/Write Timeouts Instead of switching off blocking mode, you could use stream_set_timeout() to set a timeout on read/write operations instead.
  8. Connecting to Remote Hosts Using Sockets 171 $fp = fsockopen(“tcp://www.php.net”, 80, $sock_errno, $sock_errmsg); stream_set_timeout($fp, 10); echo “Attempting to read from the stream ... this will timeout in 10 secs\n\n”; $result = fgets($fp); fclose($fp); echo “The fgets() has timed out, and returned:\n”; var_dump($result); ?> Closing a Socket When you have finished with a socket, you should close it as soon as possible. The computer that your PHP script is running on can only open a limited number of sockets.The same is true for the network server at the other end of your socket.The sooner you can close your socket, the sooner the computer’s operating system can recy- cle the network connection for someone else to use. Use fclose() to close your socket:
  9. 172 Chapter 10 Stream and Network Programming Further Reading The seminal work on TCP/IP and socket programming is the series of books written by the late W. Richard Stevens. n UNIX Network Programming Volume 1: Networking APIs—Sockets and XTI,W. Richard Stevens, Prentice Hall, ISBN 013490012X n UNIX Network Programming:The Sockets Networking API,W. Richard Stevens, Bill Fenner, Andrew M. Rudoff, Prentice Hall, ISBN 0131411551 n UNIX Network Programming: Interprocess Communications,W. Richard Stevens, Prentice Hall, ISBN 0130810819 Individual network protocols are normally documented in the Request For Comments (RFC) series published by the Internet Engineering Task Force (IETF). For more details, see http://www.rfc-editor.org/. Exam Prep Questions 1. The company you work for writes and sells a successful content management sys- tem (CMS). The CMS is written in PHP. Recently, your company has acquired the assets of one of your main competitors, including their CMS. The plan is to discontinue the rival CMS, and migrate all of its current customer base over to your CMS. However, this isn’t going to happen until you’ve added some of the features that your CMS is currently lacking. The first feature that you have to add is a dead link checker. This handy little util- ity runs from the command-line, and checks a list of URLs to see whether they still work or not. Thanks to the new streams support in PHP 4.3, this should be very easy to do. Unfortunately, the first time you test your code, this error appears on the screen: Warning: fopen(): URL file-access is disabled in the server configuration in on line 3 Warning: fopen(URL): failed to open stream: no suitable wrapper could be found in on line 3 What is the cause of this error? Choose from one of the following. A. File wrappers don’t allow you to access websites. You need to use the CURL extension for that. B. The web server is running behind a firewall, which is preventing access out to the Internet.
  10. Exam Prep Questions 173 C. The web server’s configuration file contains the setting ‘allow_fopen_url=Off ’, which prevents the PHP file wrappers from working. D. The php.ini configuration file contains the setting ‘allow_fopen_url=Off ’, which prevents the PHP file wrappers from working. The correct answer is D. 2. Now that you’ve fixed that little problem and are able to connect to remote web- sites from your PHP script, you’re faced with another problem. Your script’s job is to determine whether or not a given URL is valid. How is your script going to do that? Choose from one or more of the following options. A. If the fopen() call fails, your script can assume that the remote website no longer exists. B. Once you have opened the file wrapper, try reading from the file. If the read fails, then the remote web page no longer exists. C. Check the metadata returned by opening the file, and use the HTTP status code returned by the server to determine whether or not the remote webpage still exists or not. D. You can’t use PHP to reliably check whether remote URLs exist or not. That’s why all these tools are always written in Java. The correct answers are A and C. 3. Decoding the status code contained in the file wrapper’s metadata is an important task. Where should you look to understand what the status code means? Choose from one or more of the following: A. The PHP Manual. It’s well annotated, so even if the PHP developers forgot to list the status codes, you can be sure that a helpful PHP user has added them somewhere. B. Microsoft.com. Internet Information Server is the web server of choice for many companies. Open standards are a nice ideal, but in the real world if code doesn’t work for customers, you don’t get paid.
  11. 174 Chapter 10 Stream and Network Programming C. W3C.org. They set the standards, and standards are important. By support- ing the open standards, you can be sure that your code will work with most of the products out in the marketplace. D Apache.org. The Apache web server is more popular than all the other web servers put together. If your code works with Apache, then it supports the market leader. And that’s an important position to be in. The correct answers are B and C. 4. Your boss was so impressed with your new dead link checker tool that he’s given you responsibility for adding a larger feature to the CMS product proper. He wants you to add file replication support. For large websites, it can be very expensive to purchase a server powerful enough to cope with all the traffic and associated load. It’s often much cheaper to pur- chase three or four smaller web servers, with a more powerful server acting as the admin server. New content is added to the admin server, and then pushed out to the smaller web servers. Although most of the content lives in a shared database, original media files (PDF files, images,Word documents, and the like) are served directly off disk. This is partly a performance decision, and partly because some database servers have severe limits on their support for replicating large amounts of binary data. You must write some code to copy files from the admin server to one or more web servers. There are no firewalls between the servers. How would you do this? Choose one or more of the following options. A. Put the media files into the database, and configure the web servers to retrieve the files from the database when they are needed. B. Use file wrappers to write the media files out to a \\server\share network share. C. Don’t use file wrappers at all. Use NFS to mount the disks from the admin server on all the web servers, and access the files directly. D. Use NFS to mount the disks from the web servers directly onto the admin server. Have the admin server write to each of the NFS mounts in turn. The correct answers are B and D.
  12. Exam Prep Questions 175 5. Customers are fickle things. Just as you have your new file replication code working, one of your major cus- tomers informs you that they have installed a firewall between the admin server and the web servers. This totally prevents your file replication code from working. Helpfully, the customer does allow outgoing HTTP connections through the fire- wall. You’ll need to provide an alternative script, that uploads the files to the web servers through a HTTP connection. How are you going to do that? Choose from one or more of the following. A. File wrappers can’t upload files via http. You’ll have to use the CURL extension to achieve this. B. Just open a URL as a file and write to it. The whole point of file wrappers is to make operations like this easy. C. Use the stream context to tell the http file wrapper where to upload the file, and have a script on the web servers move the file from the uploads directo- ry to their final destination. D. Use the FTP file wrapper to upload files directly to their final destination. The correct answer is C. 6. With file replication done and dusted, your boss is confident that he’ll soon have customers migrating across from the discontinued CMS to your product. He’ll have no trouble making his targets for the quarter, and earning his bonus. However, he needs one more feature porting across before he can be certain that customers will start migrating. Many sites like to keep in touch with their customers via a weekly newsletter. Many customers only come back to the website because there was something of interest to them in the newsletter. Being able to send newsletters—and being able to make those newsletters look professional—is an important feature. Your CMS doesn’t support the concept of newsletters per se. But it does support the idea of packaging groups of files for downloading. If you could write a user- defined file wrapper that makes a MIME email look just like a ZIP file, it would then be very easy to add newsletter support. Sketch out a design for a file wrapper, which would allow a PHP script to add content, graphics, and other attachments to a MIME email.
  13. 11 Security Terms You’ll Need to Understand n Data filtering n register_globals n SQL injection n Command injection n Cross-site scripting (XSS) n Shared hosting n safe_mode n open_basedir Techniques You’ll Need to Master n Validating client data n Understanding the register_globals directive n Escaping data used in SQL statements n Escaping data used in shell commands n Preventing cross-site scripting attacks n Understanding the safe_mode directive n Understanding the open_basedir directive
  14. 178 Chapter 11 Security Data Filtering Data filtering, the process of validating data and filtering out that which is invalid, is arguably the cornerstone of Web application security.The basic premise is quite simple: Never trust foreign data, especially data from the client. There are two fundamentally different approaches to data filtering: the whitelist approach and the blacklist approach.With a whitelist approach, you assume data to be invalid unless it is proven otherwise (by meeting certain requirements of validity).With a blacklist approach, you assume data to be valid unless proven otherwise. Of course, the whitelist approach is stricter, and therefore more secure. More pertinent than the principles of data filtering are the applications of it, many of which are covered in the following sections. Register Globals In PHP 4.2.0, the default value of the register_globals directive changed from On to Off. PHP professionals are now expected to write code that does not rely on register_globals. When enabled, register_globals imports data from several different sources into the global namespace. Of particular interest to most developers is that the data from $_POST, $_GET, and $_COOKIE is available in regular global variables. For example, if a POST request contains a variable named foo, not only is $_POST[‘foo’] created, but $foo is also created. Although this behavior is simple and well documented, it carries serious implications with regard to data filtering.Whereas it is quite easy to determine that $_POST[‘foo’] is something that needs to be validated prior to use, the origin of $foo is less clear when register_globals is enabled. In addition, if variables are not properly initialized, it is possible that you might use a variable sent from the client when you intend to be using a variable that you create yourself. A common example of this mistake is as follows: if (authorized()) { $admin = true; } /* Later... */ if ($admin) { /* Sensitive activity */ } Because $admin is not properly initialized, a user can arbitrarily set its value by leverag- ing the behavior of register_globals. For example, the user can call the page with
  15. SQL Injection 179 ?admin=1 appended to the URL.This will cause $admin to be set to 1 at the beginning of the script. An important point is that a user has no control beyond the start of the script because a user can only manipulate the HTTP request. Once PHP begins execu- tion, the request has been received, and a user can do nothing more to affect the pend- ing response.This is why initializing your variables (and thereby overwriting any user-injected values) is such a good practice. Of course, with proper programming practices, register_globals does not pose a significant risk. However, having register_globals enabled makes the magnitude of a mistake much greater, and it also makes it more difficult to identify foreign data. The following guidelines are recommended, regardless of whether register_globals is enabled: n Always initialize variables n Develop with error_reporting set to E_ALL n Filter all foreign data SQL Injection When querying a database, you will likely need to use foreign data in the construction of your SQL statement. For example, when storing data, you might be using values that the user supplies in an HTML form.When retrieving data, you might be using the user’s username or some other client-supplied unique identifier as your primary key. Regardless of the reason, using foreign data in the construction of an SQL statement is something that poses a significant security risk.This cannot be avoided in most cases, but there are some best practices that can help mitigate the risk. The first step, of course, is to properly filter the data, as just discussed. Most SQL injection vulnerabilities are a result of poor, or absent, data filtering. It is unlikely that valid data is going to pose a serious security risk. With valid data, the only remaining concern is that you escape the data.This includes making sure that characters in the data aren’t misinterpreted as being part of the SQL construct. If single quotes are properly escaped, this risk can be mitigated by always enclosing the data in single quotes within your SQL statement. For example, $sql = “insert into foo values (‘$bar’)”; As long as $bar does not contain any unescaped single quotes, it cannot interfere with the construction of the SQL statement. Of course, there are other characters worth escaping, and depending on which database you use, PHP might have functions specifically designed for this task. For example, MySQL users can rely on mysql_escape_string() to do the escaping. With some databases, certain data types (notably integers) cannot be enclosed in sin- gle quotes, but the data filtering for this type of data can be much stricter so that the other safeguards are less important.
  16. 180 Chapter 11 Security Command Injection Another dangerous activity is executing shell commands whereby the user has supplied a part of the command. Mitigating this risk is very similar to mitigating the risk of SQL injection, although there are some specific PHP functions that you should learn. With properly filtered data, there are only two potential problems that you might encounter regarding shell commands: 1. There might be metacharacters that can be used to execute arbitrary commands. 2. If the data being used to construct a command is intended to be a single argu- ment, there might be characters within the data that cause it to be interpreted as multiple arguments instead. These problems are solved with escapeshellcmd() and escapeshellarg(), respectively. Data passed through escapeshellcmd() will be escaped such that it no longer poses a risk of arbitrary shell command execution. Data passed through escapeshellarg() can safely be used as a single argument. Cross-Site Scripting One of the most frequent vulnerabilities in modern PHP applications is cross-site script- ing (XSS). As with most security concerns, proper data filtering can practically eliminate the risk of cross-site scripting. However, in this case, the real risk is when foreign data is used in your output and thereby potentially displayed to other users.This is fairly typical for applications such as Webmail, forums, wikis, and even 404 handlers. The best defense of cross-site scripting is to use functions such as htmlspecialchars() or htmlentities() on data prior to displaying it. Of these two functions, htmlentities() is better for this purpose because it is more inclusive in terms of what entities it encodes. This is a blacklist approach, but because there are a finite number of well- documented characters that have a special meaning within HTML, it is actually a pretty strong approach in this case. Of course, it is still best to be strict in your data filtering. If you are expecting a person’s first name, should valid JavaScript make it through your data filtering? Hopefully you agree that this is not desirable. Other functions such a strip_tags() (that attempts to remove all valid HTML and PHP) can also help in preventing cross-site scripting vulnerabilities, but this is an exam- ple of a somewhat weaker blacklist approach than what htmlentities() provides. Shared Hosting A common dilemma among PHP developers is achieving a satisfactory level of security on a shared host.There has been some effort to resolve some of the shared hosting secu- rity concerns, but none of these can help a shared host reach the level of security that you can achieve on a dedicated host.
  17. Exam Prep Questions 181 Two particular attempts to address this problem are the safe_mode and open_basedir directives.The safe_mode directive effectively limits the files that a PHP script can open to those with the same ownership as the PHP script itself.This can help to prevent peo- ple from casually browsing the entire filesystem using a specially crafted PHP script, but it unfortunately cannot address situations in which other languages are used to achieve the same. The open_basedir directive is similar—except that instead of relying on file permis- sions, it restricts the files that PHP can open to those within a certain directory.Thus, PHP cannot be used to open files outside of the directory specified by open_basedir. One somewhat tricky characteristic of open_basedir is that you can use partial names to match more than one directory. For example, a value of /tmp/foo will match both /tmp/foo and /tmp/foobar. If you want to restrict access to only /tmp/foo, you can use a trailing slash so that open_basedir is set to /tmp/foo/. Both of these directives require administrative access, of course; otherwise, a developer could simply override these settings. Exam Prep Questions 1. Which of the following data filtering methods can be described as a whitelist approach? A. Make sure that a username does not contain backticks or angled brackets. B. Only allow alphanumerics and underscores in a username. C. Pass all incoming data through strip_tags(). D. Use htmlentities() to escape potentially malicious characters. Answer B is correct. Answer A is incorrect because this assumes that any username without backticks or angled brackets is valid. Answer C is incorrect because this only removes HTML and PHP tags, assuming everything else to be valid. Answer D is incorrect because htmlentities() only encodes HTML entities and is not intended to filter data at all. 2. With register_globals enabled, which of the following practices is particularly important? A. Initialize all variables. B. Filter all foreign data. C. Escape all data used in SQL statements. D. Escape all data prior to output. Answer A is correct. Answers B, C, and D are incorrect because these practices are not dependent on whether register_globals is enabled.
  18. 182 Chapter 11 Security 3. What are the two most important practices to mitigate the risk of an SQL injec- tion vulnerability? A. Disabling register_globals and enabling safe_mode. B. Enabling safe_mode and filtering any data used in the construction of the SQL statement. C. Filtering and escaping any data used in the construction of the SQL state- ment. D. Disabling register_globals and escaping any data used in the construction of the SQL statement. Answer C is correct.With properly filtered data, escaping any metacharacters that remain can mitigate the remaining risks. Answers A, B, and D are incorrect because register_globals does not directly affect the risk of SQL injection, and safe_mode is unrelated. 4. If $foo is anticipated to be a string, what modification made to the following query will mitigate the risk of an SQL injection vulnerability? $sql = “insert into mytable values ($foo)”; A. Specify the column name in the SQL statement. B. Remove the parentheses surrounding $foo. C. Replace the parentheses surrounding $foo with single quotes. D. Add single quotes around $foo. Answer D is correct. Answer A is incorrect because specifying the column name does not affect the behavior of the SQL statement. Answers B and C are incorrect because the parentheses are required. 5. What is the purpose of the escapeshellcmd() function? A. To prepare data so that it can be used as a single argument in a shell com- mand. B. To remove malicious characters. C. To escape metacharacters, so that they can’t be used to execute arbitrary commands. D. To prevent cross-site scripting attacks. Answer C is correct. Answer A is incorrect because escapeshellcmd() does not attempt to solve this problem. Answer B is incorrect because escapeshellcmd() does not actually remove characters. Answer D is incorrect because escaping data to protect against cross-site scripting is much different than escaping data to be used in a shell command.
  19. Exam Prep Questions 183 6. What is the purpose of the escapeshellarg() function? A. To prepare data so that it can be used as a single argument in a shell com- mand. B. To remove malicious characters. C. To escape metacharacters, so that they can’t be used to execute arbitrary commands. D To remove arguments from a shell command. Answer A is correct. Answers B and D are incorrect because escapeshellarg() does not remove characters. Answer C is incorrect because escapeshellarg() does not attempt to solve this problem. 7. When is cross-site scripting a heightened risk? A. When storing data submitted by the user. B. When displaying foreign data. C. When executing a shell command. D. When opening a remote URL. Answer B is correct.When displaying foreign data that is not properly escaped, you can inadvertently expose your users to significant risk. Answer A is incorrect because storing data poses no immediate threat, even though this might result in a cross-site scripting vulnerability later. Answers C and D are incorrect because these activities are unrelated. 8. Which of the following functions can be used to escape data such that it can be displayed without altering the appearance of the original data? A. htmlspecialchars() B. addslashes() C. escapeshellargs() D. urlencode() Answer A is correct because htmlspecialchars() will convert special characters to HTML entities that will display correctly in any Web client. Answer B is incor- rect because addslashes() only escapes single quotes. Answer C is incorrect because escapeshellargs() is only helpful when dealing with shell command arguments. Answer D is incorrect because URL encoding is not interpreted by Web clients except in the context of URLs.
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