That will probably help, though it's not an option unless you have a dedicated server (and even then, we'd probably have to install it for you as it's not installed by default).
Also, make sure you have cached as much content as possible. Re-generating something every 30 minutes and writing it to a file is usually better than re-generating it upon request if A) most site visitors are seeing the same thing, B) the content doesn't need to be absolutely fresh, and C) you're getting tons of traffic.
Finally, MySQL v4.0 introduces query caching, which can greatly speed up certain kinds of queries if the exact same query is repeated over and over. The downside is that we don't currently support MySQL 4.0, but it's definitely on the way.
As always, be sure that you are using indexes properly with your database tables. In my own projects I've seen speed-ups in the order of 10x as fast after adding certain indexes.
Also consider using persistent DB connections.
I don't think you'll find any problems with PHP itself scaling, though it's largely dependent on how your code is written and the hardware you throw under it. There are some rather high traffic dynamic sites running PHP out there. Admittedly, few of them are probably running on shared hosting, and have buff hardware as a foundation.
- Jeff @ DreamHost
- DH Discussion Forum Admin