So some well spent time digging around in the xdebug PHP extension, and more consultation with Derick have lead me to look into the executor part of the Zend engine for where I can properly redirect function calls. I'm at odds know as to how to approach this though. It's probably easiest to just wrap the zend_execute() function call like xdebug does and check the function hash table to see if a function call exists or not. But I still keep thinking about the possibility of just providing a method in the SRM object to "import" the functions from the SRM library into the running PHP SRM client's function hash table and have a handler function to redirect those calls to SRM. There's also the possibility of merging both of those approaches into a wrapper for zend_execute() the "learns" about SRM functions by checking if they exist in SRM, and if they do, add them to the function hash and then the function call handler will take care of calling them. I guess it all depends on what makes the most sense - so I guess I'll poll the mailing list and see what folks think.
So we've now officially begun selling our product at work today - which means this is the first (real) job I've had with a company that has produced, marketed, and sold an actual product. This is also the longest I've been at any job, the most fun I've had, the most learning & maturing I've done, and overall the most positive work experience so far. All thanks to the wonders of open source software such as PHP, Apache, MySQL, etc.
Two weeks until we move into our new apartment. :)