Securing AJAX requests with PHP?

I realize this isn't really about PHP, but I was hoping maybe someone had a way to make AJAX a little bit more secure using PHP. I was thinking of making my AJAX calls also pass the current session id, and have my PHP script check to make sure it's a valid id, but I'm open to other ideas. Do you guys use PHP to make AJAX calls a little bit more secure? What /do/ you use? I hope this isn't too off-topic. Thanks, Jay