What does VC9 stand for in PHP version?

I need to know what does VC stand for in the PHP version:

VC9 x86 Non Thread Safe (2011-Nov-10 19:52:12)

To expand on the other (also correct) answers a little further, from windows.php.net:

Which version (of PHP for Windows) 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.

Hope this helps.

that's Visual Studio. VC9 is Visual Studio 2008

I believe this refers to the compiler version. In this case, MS Visual Studio C/C++ version 9.

Visuall C++ wikipedia link