You will need at least version PhP 4.3.7 (older versions may work but are not tested)
There are no special PhP -modules required. A database module and zlib may come in handy. If you wish to use your own custom WebBuilder2 - ApplicationModules that do special things you may need other PhP -modules aswell.

Required php.ini Ini-Settings:
* 'magic_quotes_gpc' => false
* 'magic_quotes_runtime' => false
* 'register_globals' => false
* 'safe_mode' => false
* 'short_open_tag' => false
* 'asp_tags' => false