If you are using PHP with Apache 1 or Apache2 from apache.org you need to use the VC6 versions of PHP
If you are using PHP with IIS you should use the VC9 versions of PHP
That seems to make the decision pretty simple. If you're running Apache, get VC6, if you're running IIS get VC9. Don't stress thread safe over non-thread safe, just close your eyes and pin the tail on the donkey. They're almost identical packages, and there's some dispute over whether or not PHP is truly thread safe or if they're just fibbing to us anyways.
I apologize for the lack of help in my original post. I couldn't resist.