There is a tool that used to come with the cpanel installation on some of the bigger hosts (hostgator, bluehost, etc…) call phpFormGenerator. This tool would let you create forms in a GUI and then give you code to upload to your website.
Part of this code was a file called global.inc.php. global.inc.php included code that read the session variables that were passed to the post. At the time it was written PHP3 was in common use and a function called pt_register was included that made assigning the value from a form field to a variable a little easier (it was kind of clunky at that point).
Basically you would have something like
pt_register('POST','Foo');
This would assign the contents of form field Foo to a variable $Foo. The way this was accomplished was inherently unsafe and has been removed completely from current versions of PHP (PHP5.5).
If you have moved a site to a server running php 5.5 that uses this code the forms will be broken.
Thankfully there is an easy fix. Form field contents are now put into the $_POST variable. This means you can replace the pt_register call with this
$Foo = $_POST['Foo'];
Exact same thing is accomplished, just replace the pt_register lines in any form php files and the forms will work again.