You might have to use a global variable to access the data. Which one that you should use depends on what version of PHP you have, and which method you're using. Well, let's take them all...
<?php
# A form with the post method has been submitted and the
# data will be processed here.
# On the newest version you must use $_POST or $_GET
# depending on your method, but in the old versions you
# have to use $HTTP_POST_VARS or $HTTP_GET_VARS.
// A "Post" form
print "$HTTP_POST_VARS[firstname]";
print "$_POST[firstname]";
// A "Get" form
print "$HTTP_GET_VARS[firstname]";
print "$_GET[firstname]";
# Just print both of these, and then you'll see which one
# that is working.
?>
