And to provide a somewhat more technical answer to your question. AOLserver Tcl/ADP is more flexible than PHP, which is very much tied to an HTML-like environment. AOLserver/Tcl is thread-safe and optimized for threaded execution, and while I don't know PHP's precise thread-safety status, I'm pretty sure it's not nearly as good as AOLserver/Tcl's. Finally, there's DB pooling and connection handling in general. The AOLserver/Tcl system is much cleaner and more efficient than PHP's weak pconnect() mechanism.
However, we are not opposed to a PHP port, as mentioned by other OpenACSers. If enough people are interested, they should definitely provide this port! I think it's a tough challenge, and may not be worth the trouble, but if it's worth it to someone else, we'll certainly link to it and applaud them for making OpenACS available to still more people.
It's just that no one on this team so far feels that the "I don't want to download AOLserver" excuse is a strong enough limitation that needs to be overcome. AOLserver is a solid, truly Open-Source product that more than meets our needs and can definitely meet the needs of any PHP programmer.
