PHP Manual

Passing the Session ID

There are two methods to propagate a session id:

The session module supports both methods. Cookies are optimal, but because they are not always available, we also provide an alternative way. The second method embeds the session id directly into URLs.

PHP is capable of transforming links transparently. Unless you are using PHP 4.2.0 or later, you need to enable it manually when building PHP. Under Unix, pass --enable-trans-sid to configure. If this build option and the run-time option session.use_trans_sid are enabled, relative URIs will be changed to contain the session id automatically.


The arg_separator.output php.ini directive allows to customize the argument separator. For full XHTML conformance, specify & there.

Alternatively, you can use the constant SID which is defined if the session started. If the client did not send an appropriate session cookie, it has the form session_name=session_id. Otherwise, it expands to an empty string. Thus, you can embed it unconditionally into URLs.

The following example demonstrates how to register a variable, and how to link correctly to another page using SID.

Example #1 Counting the number of hits of a single user



if (empty(
$_SESSION['count'])) {
$_SESSION['count'] = 1;
} else {

Hello visitor, you have seen this page <?php echo $_SESSION['count']; ?> times.

To continue, <a href="nextpage.php?<?php echo htmlspecialchars(SID); ?>">click

The htmlspecialchars() may be used when printing the SID in order to prevent XSS related attacks.

Printing the SID, like shown above, is not necessary if --enable-trans-sid was used to compile PHP.


Non-relative URLs are assumed to point to external sites and hence don't append the SID, as it would be a security risk to leak the SID to a different server.

PHP Manual