PWS Registry question

This is more academic than anything else, but I was trying to study the various implementations of PHP on windows. As one test, I wanted to first configure PWS (on W98) to run PHP as a CGI, then switch over to an isapi.dll. I realize that ISAPI is still considered experimental except on W2K, and unstable at that, but this was an academic excercise. My assumption is that I just need to edit the registry entry and change it from \path\to\php.exe to \path\to\sapi\php4isapi.dll. Not only did this not work, but it "seemed" that I could change the entry to anything, and PWS would continue to run the CGI version of PHP. (I did completely reboot the system after each change) So... are my assumptions correct, or am I missing something? Thanks for any insight.