Jquery Help

Hi all..

I just wanted to keep all of my jquery code externally which is currently inside my main .php file.
Facing one challenge as below --

In my php file I have this jquery code --

$userid = mysql_real_escape_string($_SESSION['uid']);

var userid = <?php echo "$userid"; ?>; //getting used in the Ajax call

Now but when I keep this code in an external .js file then how do I get the userid in javascript variable . which is actually coming from $_SESSION Variable ..

Any clue ??