In the zips found here: http://windows.php.net/downloads/pecl/releases/apc/
There are two files, "php_apc.dll" and "php_apc.pdb" Any ideas what the .pdb is for?
You don't need the PDB file in order to use the extension.
PDB stands for Program Database, a proprietary file format (developed by Microsoft) for storing debugging information about a program (or, commonly, program modules such as a DLL or EXE). PDB files commonly have a .pdb extension.