PHP keep dropdown value after submit
<p>I have the following code for a simple calculator. The code works fine, but I want the value in the dropdown list to stay there after submitted. How would I do this? 
Essentially, I want the operator to stay selected once the calculation has been done. At the moment, it just shows a '+', even if the sum is 25 / 5.</p>

<pre><code>&lt;?php 

$number1 = $_POST['number1'];
$number2 = $_POST['number2'];
$operation = $_POST['operator'];


Switch ($operation) {
case 'add': $answer = $number1 + $number2;
break;
case 'minus': $answer = $number1 - $number2; 
break;
case 'divide': $answer = $number1 / $number2; 
break;
case 'multiply': $answer = $number1 * $number2;
break;
}


?&gt;

&lt;form name='calculator' method='post' action=''&gt;
&lt;table&gt;
    &lt;tr&gt;
        &lt;td&gt;
            &lt;input name="number1" type="text" value="&lt;?php     i    if(isset($_POST['number1'])) { echo  htmlentities($_POST['number1']);}?&gt;"         /&gt;
        &lt;/td&gt;
        &lt;td&gt;
            &lt;select name="operator"&gt;
                &lt;option value="add"&gt;+&lt;/option&gt;
                &lt;option value="minus"&gt;-&lt;/option&gt;
                &lt;option value="divide"&gt;/&lt;/option&gt;
                &lt;option value="multiply"&gt;x&lt;/option&gt;
            &lt;/select&gt;
        &lt;/td&gt;
        &lt;td&gt;
            &lt;input name="number2" type="text" value="&lt;?php     if(isset($_POST['number2'])) { echo  htmlentities($_POST['number2']);}?&gt;"     /&gt;
        &lt;/td&gt;
        &lt;td&gt;    
            &lt;input name="submit" type="submit" value="=" /&gt;
        &lt;/td&gt;
        &lt;td&gt;        
            &lt;input name="" type="text" value="&lt;?php echo $answer ?&gt;" /&gt;
        &lt;/td&gt;
    &lt;/tr&gt;
&lt;/table&gt;
</code></pre>

<p></p>