Installing Extensions

["example.email.address@xxxxxxxxx" <example.email.address@xxxxxxxxx>]

Hi

I'm wanting to install Apache 2, PHP 5.1.5 and MySQL 5 on Mac OS X 10.4.7.

When building and installing PHP, do I have to (a) build-in ALL the
extensions I want, OR (b) can I build and install PHP with the freedom
to install extensions AFTER php is already running?

Thanks
Tim