Invoking a DLL with PHP

Good morning gurus and gurettes! I have been asked (and I think that we may have discussed this before in general terms, a long time ago) to perform the preliminary analysis on a project that ideally would have me invoking an existing .DLL using PHP. I have been googling for tidbits for several days without much luck. I have not built a test, but I will be doing so later today or tomorrow. Does anyone have any insight to this type of process? Would I have to build a PHP extension and compile the DLL as part of PHP's configuration? Ideally the DLL would be a black box that could be accessed utilizing functions that I build without a re-compile of any type. Thanks for any insight that you may have...all clues welcome!