Here's a link to some slides from Yahoo's Director of Engineering describing their experience using php and mysql. And if anyone ever questions whether either php or mysql will hold up under a big load, consider that the site he describes gets something like 1.9 Billion (1,900,000,000) pageviews per day :eek:
http://public.yahoo.com/~radwin/talks/one-year-of-php-oscon2003.htm