I kept on getting a problem with Apache starting up that prevented it from including the MS sql server pdo drivers.
The message was:
the program can't start because msvcp110.dll is missing from your computer. Try reinstalling the program to fix the problemand this usually relates to the Visual C++ Redistributable for Visual Studio 2012 package not being installed.
I had previously installed this (it's available on the Microsoft site) but was still getting the error.
Eventually I stumbled on this topic on the Apache friends forum which advised to copy the msvcp110.dll file to both
the \xampp\apache\bin and the \xampp\php directories.
Apparently Apache wasn't able to follow the OS path to find the file. In my case it was already in the php directory but not with the other binaries for Apache.
After copying it there Apache restarted without errors and the PDO driver for MS-SQL was enabled.