A little bit of your code would make your need clearer, anyway, try something like
<?php
if (isAdminstrator($user))
$desiredPage = 'adm.php';
else
$desiredPage = 'reg.php';
echo "<form method=\"POST\" action=\"$desiredPage\">";
?>
maybe you missed method, or maybe you missed "s (maybe both ;P)
hope it hekps,
Maxxx