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.
Note:
The arg_separator.output php.ini directive allows to customize the argument seperator. 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
<?php
session_start();
if (empty($_SESSION['count'])) {
$_SESSION['count'] = 1;
} else {
$_SESSION['count']++;
}
?>
<p>
Hello visitor, you have seen this page <?php echo $_SESSION['count']; ?> times.
</p>
<p>
To continue, <a href="nextpage.php?<?php echo htmlspecialchars(SID); ?>">click
here</a>.
</p>
The htmlspecialchars() may be used when printing the SID
in order to prevent XSS related attacks.
Printing the SID, like shown above, is not necessary if
--enable-trans-sid was used to compile PHP.
Note:
Non-relative URLs are assumed to point to external sites and hence don't append the
SID, as it would be a security risk to leak theSIDto a different server.
Anonymous (2010-03-16 22:23:32)
The first time a page is accessed, PHP doesn't yet know if the browser is accepting cookies or not, so after session_start() is called, SID will be non-empty, and the PHPSESSID gets inserted in all link URLs on that page that are properly using SID.
This has the consequence that if, for example, a search engine bot hits your home page first, all links that it sees on your home page will have the ugly PHPSESSID=... in them.
This appears to be the default behavior. A work-around is to turn on session.use_only_cookies, but then you lose session data for anyone who has their cookies turned off.
CT Thompson (2009-07-17 13:17:48)
The previous user is correct, but you can get around this by starting your sessions like this:
session_start($mysession);
Or you can set up an if statement that determines if $mysession is set and whatnot...
a dot nielsen at shikadi dot net (2009-02-09 18:49:55)
When you generate URLs yourself using the SID variable, it must come first in the URL parameter list. For example, if SID expands to "mysession=123" then this does NOT work:
file.php?var1=value&mysession=123
The SID must come first:
file.php?mysession=123&var1=value
If the SID does not come first then session_start() does not recognise it and it creates a new session instead of loading the old one.