how to write absolute path for category archive?

my current dropdown category list works on the front page, but for obvious reasons, it links incorrectly if it’s not on the main page.
i’m using this code:
<form id="categoryform" name="catform" onchange="window.location = 'index.php?cat='+(document.forms.catform.cat[document.forms.catform.cat.selectedIndex].value);" action="<?php echo $PHP_SELF ?>" method="get" >
<?php dropdown_cats(); ?>
i know the fix is easy.. i just don’t know exactly what it should be. i need it to go to an absolute path to find ‘index.php’, right? how would i write that?
thank you..