Watch for uninitialized variables
<?php
    if($user=='rasmus') {
        $ok = true;
    }

    if($ok) {
        echo "$user logged in";
    }
?>
Catch these by setting the error_reporting level to E_ALL. The above script would generate this warning (assuming $user is set):

Warning: Undefined variable: ok in script.php on line 6
You can of course also turn off register_globals, but that addresses the symptom rather than the problem.