Passing the Session ID

There are two methods to propagate a session id:

  • Cookies
  • URL parameter

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. If the run-time option session.use_trans_sid is 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.

add a note

User Contributed Notes 2 notes

14 years ago
The first time a page is accessed, PHP doesn't yet know if the browser is accepting cookies or not, so after session_start() is called, SID will be non-empty, and the PHPSESSID gets inserted in all link URLs on that page that are properly using SID.

This has the consequence that if, for example, a search engine bot hits your home page first, all links that it sees on your home page will have the ugly PHPSESSID=... in them.

This appears to be the default behavior. A work-around is to turn on session.use_only_cookies, but then you lose session data for anyone who has their cookies turned off.
EastWhite at foxmail dot com
8 years ago
The constant SID would always be '' (an empty string) if directive session.use_trans_sid in php ini file is set to 0.
But set session.use_trans_sid to 1 is not effective if director session.use_only_cookies is 1.
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