Update Syntax and Passing Strings - php

Question

I'm running into an issue while using the Update syntax when updating a table row with a string. I'll post the code below then explain my issue:
$query = "SELECT * FROM foo WHERE bar = '$id'";
$results = mysql_query($query);
$rows = mysql_num_rows($results);
for ($j = 0 ; $j < $rows ; ++$j) {
$entity_id=mysql_result($results,$j,'entity_id');
}
//$entity_id = "129";
if ($_POST['submit']) {
$text = mysql_real_escape_string($_POST['comments']);
$query = "UPDATE field_data_body SET body_value='$text' WHERE entity_id='$entity_id'";
mysql_query($query) or die ('Error updating database' . mysql_error());
}
?>
<form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
<textarea name="comments">Example Comment</textarea>
<input name="submit" type="submit" value="submit" />
</form>
Here's a quick rundown of what the code does. I first request for the value of $entity_id from the table foo. I then update a different table row using a text-area and submit button.
My issue lies on line 12:
$query = "UPDATE field_data_body SET body_value='$text' WHERE entity_id='$entity_id'";
If $entity_id is passed directly to this line, the row will not update. If I assign an integer to $entity_id, (line 8 without the comment), the row will update. I've checked to make sure $entity_id is assigned a value before it gets to the submit section of the script; it returns 129, (echo "$entity_id"; output is 129). Why can I not pass the string without having to assign an integer?
EDIT: I found where the issue was taking place. The issue was on line 18, where the action attribute is called. I removed the php, and replaced it with a #. I'm not entirely sure why this was a fix to the problem, but there you have it. Thanks to those below with their suggestions and help, I do appreciate it!