When PHP tries to insert the Session-ID into an _empty_ form action or empty href the output totally get's messed up.

Try the following:

<?php
    session_start();
    echo "<form action=\"\">";
?>

The output looks like this:

<form action="">
?PHPSESSID=xxxxxxxxxxxxxxxxxxxxxx

You can see this at http://preview.yaps.net/test_saar.php

If you output more than just a single form action,  the output partially get's repeated.