There have been other articles on this, but I wanted to post my own approach, just because it’s such an ugly — yet effective — hack.
I wanted PHP 4 and PHP 5 to both live happily on my Windows 2k machine, and work under Apache 1.3x. Install them in c:\php4 and c:\php5, and there you go. It’s easy to create an httpd.conf that takes a define, and figures out which PHP to load based upon that define.
But then both PHPs want to use the same php.ini file, specifically, c:\WINNT\php.ini. This is only a problem when you’re using PHP extensions, because one version will try to load the wrong ones. There are registry tricks, supposedly, and environment variable tricks to get different .ini files going, depending on version. None of ‘em worked for me.
So I opened up vim, and edited my php4ts.dll, and replaced the “php.ini” string with “ph4.ini”. Now I happily have two PHP installs, each with its own .ini file, and everything is copacetic — except for all that software that breaks the newly enforced reference rules. But that’s another story.