passes variable to table but cannot use it before - php

Question

I got the following piece of code in php:
I have a page called post.php which posts successfully all data using jquery. The data that it posts are: $comments_id, $reply and $the_session_user_id as you can see in my code and I can get them successfully. My problem is that my equation if($session_user_id == $the_session_user_id ) is not working. It is like it cannot get $the_session_user_id. How can this be possible, since next it pass it to the table using the mysql statement? If I delete the if($session_user_id == $the_session_user_id ) and not make this choice, it passes succesfully all data to table. Any idea why my if statement not working?
<?php
// $session_user_id is the user id of the session
// the following data posted from a page called post.php wich posts them using jquery succesfully
$comments_id = $_POST['data_comments_id'];
$reply = $_POST['data_reply'];
$the_session_user_id = $_POST['data_session_user_id'];
if(isset($reply) && !empty($reply)){
if($session_user_id == $the_session_user_id ){
mysql_query("INSERT INTO comments_reply VALUES ('', '$comments_id', '$reply', now(), '', '$the_session_user_id') ");
}else{
mysql_query("INSERT INTO comments_reply VALUES ('', '$comments_id', '$reply', now(), '1', '$the_session_user_id') ");
}
}
?>

Answer 1

It doesn't seems like $session_user_id isn't declared/assigned any value. $session_user_id will therefore be 'null'. You must first assign your $_SESSION['your_id_key'] to $session_user_id: $session_user_id = $_SESSION['your_id_key']
You can try to echo both values ($session_user_id and $the_session_user_id) before the if to see wich value they've got assigned:
echo $session_user_id;
echo $the_session_user_id;