I'm really lost here while trying to send a session with my jquery ajax post call, here is a simplified example of my code.

File fom which request is sent:

<?php
    session_start();
    $token = md5(rand(1000,9999));
    $_SESSION['contactToken'] = $token; 
?>

<script type="text/javascript">
    $.post(ContactUrl,{req:"contact_sub",tok:"<?php echo $token; ?>"},function(contactAns){
        alert(contactAns); return false;
    });
</script>

File request is sent to:

<?php
if(@isset($_SERVER['HTTP_REFERER']) && $_SERVER['HTTP_REFERER']=="url"){
    if( isset( $_SERVER['HTTP_X_REQUESTED_WITH'] ) && ( $_SERVER['HTTP_X_REQUESTED_WITH'] == 'XMLHttpRequest' ) ){

        session_start();
        $token = $_POST['tok'];
        $sess_token = $_SESSION['contactToken'];

        if($token == $sess_token){
            echo "sessions match"; exit();
        }
        else{
            echo "sessions does not match"; exit();
        }
    }
    else{echo "error"; exit();}
}
else{echo "error"; exit();}
?>

At first the session was not getting recognized, I made all the checks - made sure it was setup in the first place made sure it was posted, declared session start on both pages, never the less if i tried to do this on the second file:

<?php
session_start();
$token = $_POST['tok'];
$sess_token = $_SESSION['contactToken'];

print_r($_SESSION['contactToken']); exit();
?>

I would get an empty alert. Then I transferred the session start to the top of my script on the second page and started getting a value for the session:

<?php
session_start();
$sess_token = $_SESSION['contactToken'];

if(@isset($_SERVER['HTTP_REFERER']) && $_SERVER['HTTP_REFERER']=="url"){
    if( isset( $_SERVER['HTTP_X_REQUESTED_WITH'] ) && ( $_SERVER['HTTP_X_REQUESTED_WITH'] == 'XMLHttpRequest' ) ){

        $token = $_POST['tok'];
        echo "$token, $sess_token"; exit();

    }
    else{echo "error"; exit();}
}
else{echo "error"; exit();}
?>

And what I'm getting now is that the posted variable changes each time I refresh the page but the $sess_token always gives me the same value: 0589dd536fd043ff3865f8223fef3030

I really dont understand this wierd behavior, can some one please assist me with this?

Your problem here is that you're using a PHP var in an JS script without wraping and echoing it.. Here is your code modified:

You're also trying to contatenate with . in JS. That's from PHP too.

<script type="text/javascript">
    $.post(ContactUrl, {
        req: "contact_sub",
        tok: "<?php echo $token; ?>"
    }, function(contactAns) {
        alert(contactAns); 
        return false;
    });
</script>

Update

I came back to this answer again today. This is what I did:

FILE: index.php

<?php
    session_start();
    $token = md5(rand(1000,9999));
    $_SESSION["contactToken"] = $token; 
?>

<script type="text/javascript">
    $.post("myOtherScript.php", {
      req:"contact_sub",
      tok:"<?php echo $token; ?>"
    }, function(contactAns){
      alert(contactAns);
      return false;
    });
</script>

FILE: myOtherScript.php

<?php
   session_start();
   $sess_token = $_SESSION["contactToken"];

   if(isset($_SERVER["HTTP_X_REQUESTED_WITH"]) && ($_SERVER["HTTP_X_REQUESTED_WITH"] == "XMLHttpRequest")){

      $token = $_POST["tok"];
      echo $token ." - ". $sess_token;

   } else {
      echo "Not an AJAX request";
   }
?>

What I get is the alert where one token is equal to the other and both are refreshed each time I reload the index.php file.

As a conclusion, your problem is not in the code you shared.