Which version do I choose?

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

VC6 Versions are compiled with the legacy Visual Studio 6 compiler

VC9 Versions are compiled with the Visual Studio 2008 compiler and
have improvements in performance and stability. The VC9 versions
require you to have the Microsoft 2008 C++ Runtime (x86) or the Microsoft 2008 C++ Runtime (x64) installed

Do NOT use VC9 version with apache.org binaries

VC9 versions of Apache can be fetched at Apache Lounge. We use their binaries to build the Apache SAPIs.