namecheap restricts api calls to 30/sec, so you wouldn't be able to have a single process achieve 100/sec. I can easily do what you want with that restriction in mind for a single process (and to achieve 100/sec, you can do the same on another IP and another login, or another provider).
I've done many such highly efficient client processes before (and server processes). I wouldn't recommend PHP to do anything at the millisecond level because the scalability just isn't there. I will do a lower level multi-threaded C application for you. It's easier for me that way, and easier for you to support in the future because dealing with a single good program takes much less effort than dealing with many script-type programs.
(And by the way, if you want this efficiency with the mysql database, then you'll want the programmer to do the C-interface to the database anyways. I've got years experience doing these things).