Don't really need a threadsafe driver, as the external driver will work, just as is done with Solid and Sybase. A threadsafe internal driver would be more efficient, but the penalty's not nearly as bad as it used to be (the AOLserver gang tracked down a problem and were able to achieve a 10x speedup a few months ago).
So I would very much doubt that the driver would be a bottleneck other than for the most high-volume sites. AOL and Digital City successfully ran for years with the unfortunately slow previous version of the Sybase driver (and I'm sure run even better with the 10x faster version). I think you're raising a bit of a red-herring.
As far as PHP, this project (OpenACS) is single-focused on the ACS. General issues such as whether or not to use AOLserver+(Tcl? Python?) or Apache+PHP is are probably better addressed in the general AOLserver discussion forums at aolserver.com. Among other things, the AOLserver developers hang out there, not here, so you'll get more educated answers.
