4
I was not initially aware that VC9 and VC11 represented the Visual C++ 2008 and 2011 versions respectively until I just read it on php.net.
A discrepancy had arisen from knowing that VC9 has been around since PHP 5.3, but only 5.5 appears to "require" the runtime libray. So I am guessing that it always was part of past distros, but 5.5 omits the VC++ run-time libraries possibly due to Microsoft licensing and use policies, leaving you responsible to download and install yourself.
Another discrepancy arises from knowing that PHP for Linux/Apache also used the same VC9 designator, which raises the question.... what a Microsoft run-time library would have to do with the Linux world?
Typically in a Windows IIs implementation, a VC9 non-thread-safe version of PHP would be used, and under Apache, a VC9 thread safe version instead (regardless whether you were running Apache under Linux or Windows).