PHP won't be interpreted on a MySQL server.
PHP scripts are interpreted on a PHP server, which may also be a MySQL server.
In any case, you may be aware that generally, a PHP script executes within miliseconds and the resultant data is then passed onto a webserver and onto the client while the PHP service goes idle or parses another script.
On Apache, multiple PHP service threads can be run simultaneously, meaning that many scripts can be interpreted and thus many more clients served in parallel.
I don't have exact figures, but generally the top figure is based on your hardware and OS configuration.
