We, the MOD Team, have decided to allow php4 functionality, up to and including 4.1.0, but nothing later for MODs submitted to the MOD Database. This is because phpBB 2.0.13 uses some php 4 only code and we want MODs to be written for the same version of php that phpBB requires.
This means you can use any functions or other things in php that are included in php 4.1.0. If you require anything higher or anything that isn't part of the default php installation (GD Libraries, Imagemagick, etc) you need to say so in the author notes.
We will not be allowing the short version of variables ($_POST, $_GET, etc). This is so that MODs stay consistent with the way phpBB is coded.