$_GET not working for SQL query - php

Question

I'm trying to use both $_GET and $_POST in an sql query. The following is my code:
<?php
$assignment = mysql_real_escape_string($_GET['name']);
echo "$assignment <br>";
if (isset($_POST['add'])) {
$user = $_POST['username'];
$text = $_POST['comment'];
$query = "INSERT INTO comments (user, text, assignment) VALUES ('$user', '$text', '$assignment')";
mysql_query($query) or die('Error, comment failed to post');
}
?>
<h1>Add Comment</h1>
<form action="log_entry.php" method="post">
Name:<br/>
<input type="text" name="username" value="" />
<br /><br />
Comment:<br />
<textarea style="height:200px;" type="text" name="comment" value="" ></textarea>
<br /><br />
<input type="submit" name="add" value="Add Comment" />
</form>
However, the $assignment variable does not work in the query. It is echoed properly before the query is made but its value inside the table after the INSERT is completed is empty. What exactly is causing this?

Answer 1

Instead of trying to combine GET and POST, use a hidden input field:
<?php
$assignment = mysql_real_escape_string($_POST['name']); // Name is now in POST data, so swap this
echo "$assignment <br>";
if (isset($_POST['add'])) {
$user = $_POST['username'];
$text = $_POST['comment'];
$query = "INSERT INTO comments (user, text, assignment) VALUES ('$user', '$text', '$assignment')";
mysql_query($query) or die('Error, comment failed to post');
}
?>
<h1>Add Comment</h1>
<form action="log_entry.php" method="post">
<!-- Add hidden input to carry the name -->
<input type="hidden" name="name" value="<?php echo $_GET['name']; ?>"/>
<!-- Rest of the form is the same -->
Name:<br/>
<input type="text" name="username" value="" />
<br /><br />
Comment:<br />
<textarea style="height:200px;" type="text" name="comment" value="" ></textarea>
<br /><br />
<input type="submit" name="add" value="Add Comment" />
</form>