need for $_POST[''] changed after server upgrade

Hello All, We just upgraded our server at work, and one client's web site stoped working. I didn't write the code, I just get to clean up someone else's mess. :) After a little troubleshooting, I found that forms refered back to the same script. so far, so good. very normal. The strange part was the variable names were just "$foo" not, "$_POST['foo']". After the upgrade, the variable "$foo" was not recognized when the form was processed, so of course, everything failed. My solution was to put if (isset($_POST["foo"])) {$foo=$_POST["foo"]}; at the top of the script. This seems to make everything work. My two questions are: first, is there a better way to fix this, and second, how did this work in the first place? TIA Ray