What apache modules are required?

How can I determine what add-ons to apache will be required for my php code? I have legacy php code that was setup a long time back (and we dont have the documentation on what was done at that time). I need to get this application working on another new server, but apache has yet to be installed.

I would like to install only those components which are necessary.

a php module, pal
to run php code you need mod_php installed. That's it.

if you're talking of PHP extensions, that's another matter and it's hard do tell. I'd make it this way: log all errors and watch for "undefined function" ones. And turn appropriate extensions on.

It have to be done anyway.
Legacy code being run on fresh PHP installations usually flood your logs with errors.