custom block keeps unmarking my php code !!

hi all

iv made a custom drupal block that displays a form, it has been working fine up until today where i checked and see that it is marking my php code out by itself using the code "<--? and --?>"

Even when i edit the code and take the markings out and save it, it goes back to normal!!! i have made sure PHP is selected!! any ideas?

my code is below (this is the code drupal keeps changing it to

<div class="popuplogincustom_main">
<div class="popuplogincustom_left">
<!--?php print drupal_render(drupal_get_form('user_login')); ?-->
<div class="popuplogincustom_fb" --="">
<a href="/user/login"><img src="../images/popupfacebook.png"></a>
</div>
</div>
<div class="popuplogincustom_right">
<a href="/user/register"><img src="../images/popupregister.png"></a>
</div>
</div>