It's incredibly rare for the Internals crew to ever consider breaking backwards compatibility, but some of the most important changes in PHP 5.4 do just that by removing old "features." None of these changes should impact modern PHP code. If somehow you get bitten by any of these changes, chances are that your code dates from the PHP 4 era.

Included in his list of updates/removals/improvements are things like the full removal of safe_mode, dropping register_globals, pulling out call time pass by reference and the removal of the session registration methods.