php message Warning: Missing boundary in multipart/form-data POST data in Unknown on line 0
<p>This is my javascript</p>

<pre><code>function ajax_post(){
            // Create our XMLHttpRequest object
            var hr = new XMLHttpRequest();
            // Create some variables we need to send to our PHP file
            var url = "LiveUpdate.php";
            var sb = document.getElementById("LiveUpdate").value;
            var FirstName = document.getElementById("FirstName").value;
            var images = document.getElementById("images").value;

            var vars = "update="+sb+"&amp;FirstName="+FirstName+"&amp;images="+images;
            hr.open("POST", url, true);
            // Set content type header information for sending url encoded variables in the request
            hr.setRequestHeader("Content-type", "multipart/form-data");
            // Access the onreadystatechange event for the XMLHttpRequest object
            hr.onreadystatechange = function() {
                if(hr.readyState == 4 &amp;&amp; hr.status == 200) {
                    var return_data = hr.responseText;
                    document.getElementById("message").innerHTML = return_data;
                }
            }
            // Send the data to PHP now... and wait for response to update the status div
            var formData = new FormData(document.querySelector("form"));
            hr.send(formData); // Actually execute the request // Actually execute the request
            document.getElementById("message").innerHTML = "processing...";
        }
            //show image before uploading
                var loadFile = function(event) {
                var output = document.getElementById('output');
                output.src = URL.createObjectURL(event.target.files[0]);
                };//end image
</code></pre>

<p>and this is the form. javascript and the form is in the same file. the file name is sample.php</p>

<pre><code>&lt;form action="popup.php" method="post" id="myForm" name="myForm" enctype="multipart/form-data"&gt;
    &lt;div id="message"&gt;&lt;/div&gt;
    &lt;img src="" id="output" /&gt;
    &lt;input type="file" class="filestyle" data-buttonBefore="true" accept=".png, .jpg, .jpeg" onchange="loadFile(event)" name="images" id="images"&gt;
    &lt;input class="form-control" type="text" placeholder="First Name" name="FirstName" id="FirstName" value=""&gt;
    &lt;a href="#" class="btn btn-success" role="button" name="update" id="LiveUpdate"  onclick="javascript:ajax_post();"&gt;Update&lt;/a&gt;
&lt;/form&gt;
</code></pre>

<p>the idea in this code is. i want to insert the basic information like FirstName and Image into the database live without refreshing the page. the reason why i choose  tag to submit the ajax. so the page url Account.php?Edit=1 will not change to Account.php because if it change to account.php the popup editing mode will be close. the problem in this code is. i don't know how to fix the error <code>Warning: Missing boundary in multipart/form-data POST data in Unknown on line 0</code> can anyone help me how to fix this. thank you!</p>