Re: What PHP modules/extensions required in SMF?

At the very minimum, you'd only need mysql. SMF is old enough it doesn't use mysqli.

If you want to use the built-in image verification, you'll want gd (not required, but it works better with). It's recommended to use verification questions as image verification is easy to break nowadays.

If you want to send mail using SMTP and use SSL, you'll need the imap extension (I believe). If you're using the built-in "mail()" functionality, you do not.

Likewise, I believe using any language that is complex script or using Unicode (UTF-8) will require mbstring.

The built-in spell checker uses pspell, but most browsers have much better spell checking support.

I recall that in very few circumstances, SMF may try to use sockets, but does not rely on it.

Likewise, I am not sure if SMF uses the zip module, but if you're installing modifications, it should probably be installed.

That's it, as far as I can recall. It should be mostly accurate as of SMF 2.0. I do not know if 2.1 requires different/more.