I'm not actually talking about PHP I'm only talking about how thread intensive databases are in general and MySQL is in particular. Almost every database operation generates several threads and as the number of concurrent users of a database grows the number of threads grows very rapidly. OS X has relatively heavy thread creation costs and thread locking is relatively coarse so more competition between threads occurs. It has improved with each OS rev and I'm sure 10.5 will be no different but the ugly reality is that Linux - coming from a multi-user Unix background - provides better threading/locking than OS X coming from a single-user NEXTStep background does.
There were several tests demonstrating this around the time the 970/G5 shipped. The only article I could dig up in a pinch was
this anandtech.com one. It isn't the most technical examination but the results are clear enough. Yellow Dog Linux running on an Xserve delivered significantly better performance on the same hardware as OS X did as the number of concurrent users rose.
I love OS X to death - and serve many databases from it - but it is not the best pure database serving platform if you are worried a rising number of users. I always try to advocate best tool for the job.
Again - my $.02,
=Tod