What are the *design* options relating to distributing operations in PHP ?
PHP is going to be coupled with Java, because PHP is very popular in the presentation layer, but its API to develop business code and objets is very very ugly, and distribution capabilities of business code are... none.
Serving millions of concurrent users has a price of performance because distribution is necessary, HTTP load balancing is not the solution when you must be able to manage sessions (usual), because the machine have be the same inside a session (except that you have distributed session capabilities in your JSP server), with EJB you can distribute the load of the each session to other(s) machines.
PHP is good to amateur sites with simple web applications and few concurrent users. But not to professional sites well designed (without the crap of PHP API) and with heavy load requirements.