On my machine at work, I can handle about about 30 to 40 page views a second at anywhere from 1 to 500 simultaneous connections. That's a dual PII 750 with 512Meg ram, spinning UltraWide SCSI drives. My older box could do about 10 page views a second, with at most about 80 simo connections. It was a K6-2-300 with 128 Meg RAM and a pair of IDE hard drives in PIO mode due to a buggy motherboard driver in an older version of the linux kernel it ran on.
The only reason we retired the old server was because it was over two years old, and we usually retire or physically rebuild all servers at about 3 years for reliability purposes anyway. The old machine bever saw a load that was even 10% of what it was tested against.
The new machine, by the way, doesn't even hit more than 10% total CPU load until it has 100 simultaneous connections running with about 400 keepalives. :)
Average page size around 5k to 60k.
This is on database driven pages, by the way, like the company phonebook and discussion areas.
Real scalability comes from the ability to put more than one machine on the job, and having (a) fast backend database(s).
With apache in reverse proxy mode or a nice cisco load balancing router, you can add just about as many machines as you have bandwidth for.
If you write a database dependent site, you will likely be bound by database performance and cost, not PHP.
I'd say Apache/PHP scales better than most of the more expensive systems out there like Domino, WebSphere, IIS/VB/ASP, especially since you can scale it to dozens of boxes with no more licenses than you started with.
