PHP / MySQL call ID issue

Hi

I wonder if someone can help me with an issue that has left me scratching my head...

I have a simple page which appears as a pop-up which calls data from a MySQL database, e.g. http://www......./popup.php?id=5 (this would call the database entry with ID number 5).

The code on the page is as follows (slightly simplified for here):

<?php
include('../config.php');
$sql = "SELECT * FROM `$filename` WHERE id = '$id'";
$sql_result = mysql_query ($sql, $connection ) or die ('request "Could not execute SQL query" '.$sql);
while ($row = mysql_fetch_assoc($sql_result)) {
if ($row["content"]<>'') {
echo nl2br(stripslashes(html_entity_decode($row["content"]))).'';
};
};
?>

The problem that I appear to have is WHERE id = '$id'. If I add a number manually and do a test (e.g. WHERE id = '5') it sill show the 'content', but not if is calling the content from the id included in the URL. Can anyone point me in the right direction?

Thanks in advance
Neil