Re-compiling PHP

First of all, depending on your distro/environment, the module may be available as an extension DLL/Linux package which is much easier to install, and doesn't require any recompilation. However, if you absolutely have to...

1 - Yes, generally you'll need to specify everything from the initial build. Unless you want to radically change the setup you have (ie remove modules etc)

2 - Create a php file with the following in:

<?php
  phpinfo();
?>

View this script in a browser to see information about your current installation of PHP. At the top you should see the configure command used in the initial build. Note that the paths of some libraries may be different on your machine, if you haven't compiled before (eg paths used during compilation by your favourite distro people).

3 - Yes, generally you'll need to do ./configure [options] && make && make install to get the binaries and modules in the correct places with the configuration options you supplied.

You can compile extensions as shared objects which means you don't need to compile your previous PHP binary from scratch again. However, if for some reason, you don't want to do that, yes, you'll need to remember your previous compile time options. However, that's easily found. A quick script with the following in it, gives you your compile time options

<?php phpinfo(); ?>

My advice is to go the shared extensions route however.