For three days I had nearly despaired when I tried to extend a WordPress site with a self-written login script. Everything worked properly on my local server, but as soon as I had uploaded it, the session variables didn’t seem to be passed any more: after the login, I stayed logged in for only one more click and then got logged out again.

After several unsuccessful Google searches (with the wisdom of hindsight it’s quite easy to google, but try and find something about this issue without reading any further than this) I found the reason at last, namely the function wp_unregister_GLOBALS() inside the file wp-settings.php. Here it is:

function wp_unregister_GLOBALS() {
    if ( !ini_get('register_globals') )
        return;

    if ( isset($_REQUEST['GLOBALS']) )
        die('GLOBALS overwrite attempt detected');

    // Variables that shouldn't be unset
    $noUnset = array('GLOBALS', '_GET', '_POST', '_COOKIE', '_REQUEST', '_SERVER', '_ENV', '_FILES', 'table_prefix');

    $input = array_merge($_GET, $_POST, $_COOKIE, $_SERVER, $_ENV, $_FILES, isset($_SESSION) && is_array($_SESSION) ? $_SESSION : array());
        foreach ( $input as $k => $v )
            if ( !in_array($k, $noUnset) && isset($GLOBALS[$k]) ) {
                $GLOBALS[$k] = NULL;
                unset($GLOBALS[$k]);
            }
}

What does this function do?

• Line 27/28:
  If register_globals is not activated in the PHP settings of the web server, it won’t do anything at all (which explains the different behaviours of my script on my local and the live server). If, however, register_globals is active, it proceeds as follows:
  • Line 34:
    As the comment already says, all global variables listed here will not be reset. As you can see, $_SESSION is not being mentioned!
  • Line 36-41:
    Now, all global variables are being shifted and deleted one by one.

In short: If register_globals is activated, $_SESSION will be deleted on every single page view. No surprise that we can’t get at our session variables any more!

What’s it all about?

Well, for one thing, WordPress assumes that it’s meant to take care of the complete site – after all it is some kind of content management system. And that includes the administration of all variables as well.

What’s much more important though, is the fact, that the use of register_globals has been deprecated for a long time and the feature will even be completely removed in PHP 6. Today, it is strongly recommended not to use these kind of variables, because they involve safety hazards. So, WordPress is obviously trying to imitate the behaviour of a deactivated register_globals in order to ensure safety and avoid potential variable conflicts.

Remedy

After having solved this riddle, the solution was perfectly obvious: I have to deactivate register_globals on the web server, so WordPress wouldn’t even need to execute that stupid function. For this purpose, simply place a text file called php.ini inside your WordPress directory, containing the following line:

register_globals Off

Further information on the subject can be found on php.net:

• How does the PHP directive register_globals affect me?
• Using Register Globals
• Superglobals