PHP Session problem

Hi, I have a form which uses a PHP session to pass data from one page to another. The problem I'm having is that other users are picking up a session variable when they are also filling the form out at the same time. Users are are asked to provide their unique ID. File 1 has the following PHP session code at the top of the file:

<?php
session_start();

session_cache_limiter('private, must-revalidate');
?>

Declare the variable:

$_SESSION['some_data']=$some_data;

Upon submission the next page, File 2 has the same PHP code at the top:

<?php
session_start();

session_cache_limiter('private, must-revalidate');
?>

Recall the variable:

$some_data=$_SESSION['some_data'];

Kill the session at the base of File 2:

<?php
session_destroy();
?>

Is there a way of assigning a unique session ID to each user so that the $some_data variable is associated with the user?

Any help would be greatly appreciated.