Thanks to Perry Lewis who sent in this guide to getting up and running with PHP5, MySQL, Apache2 on WinXP:

Here’s what had to be done to getting it working on Windows XP:

Server Configuration:
Windows XP
Apache 2.0.48
PHP 5.0 Beta 3 (running SAPI DLL php4apache2.dll – PHP.ORG states they will rename the file to php5apache2.dll once PHP 5 is released for production)
MySQL 4.0.16

Steps to Setup:

1. After the initial installation and configuration of PHP 5, copy ALL DLL’s in PHP ‘dlls’ directory/folder to Win/Systems32 (especially libmySQL.dll).
2. Verify that the size of the file (libmySQL.dll) in System32 is the same as in ‘dlls’ directory/folder (this was what was causing my problems – for some reason the libmySQL.dll did not move to the System32 directory gracefully – it was ‘oversized’ for PHP 4 and ‘undersized’ for PHP 5 – which prompted me to delete the file and move it again – second time was a charm).
3. Ensure that php_mysql.dll is in the directory/folder that contains your extensions
4. Edit the php.ini file such that the php_mysql.dll IS NOT commented out in the EXTENSIONS section.

That should get it working; however, if you use as PHP.ORG now calls them ‘old style input arrays’ (e.g. – HTTP_GET VARS or its’ cousins) you need to edit the PHP.INI.  In exact, find register_long_arrays and change the default value of OFF to ON.