Passing Session IDs in the Query String

So far you have relied on a cookie to save the session ID between script requests. On its own, this method is not the most reliable way of saving state because you cannot be sure that the browser will accept cookies. You can build in a failsafe, however, by passing the session ID from script to script embedded in a query string. PHP makes a name/value pair available in a constant called SID if a cookie value for a session ID cannot be found. You can add this string to any HTML links in session-enabled pages:

 <a href="anotherpage.html?<?php print SID; ?>">Another page</a> 

It will reach the browser as

 <a href="anotherpage.html?PHPSESSID=08ecedf79fe34561fa82591401a01da1">Another page</a> 

The session ID passed in this way will automatically be recognized in the target page when session_start() is called, and you will have access to session variables in the usual way.