Is VC9 thread safe v5.3 the same as VC6 thread safe v5.3, little bit confused, having a blonde moment???
Have a read of the following…
http://stackoverflow.com/questions/1623914/what-is-thread-safe-or-non-thread-safe-in-php
Difference between PHP thread safe and non thread safe binaries | IIS Aid
VC9 Thread Safe is the same as VC6 Thread Safe. The difference VC9 is compiled with a much newer (Visual Studio 2008) compiler. This gives a lot more benefits on Windows, performance and stability. VC6 is from 1998 has never been updated from that point on, other then security patches.
There is also a group of people out there already compiling PHP for Windows using VC10 (Visual Studio 2010) as well.
Thread safe is for PHP being used as an Apache module. Non-thread safe is for PHP being used as a CGI process.
The first is thread based, the second is process based.
Which one you use depends on your setup.