i would also suggest ab as gellyfish has

another apache offering is the new apache test platform , a sub project of apache itself
http://httpd.apache.org/test/

in there you will find flood , afaik ab next gen, and the perl framework, i think you should be able to devise some perl tests that can test your php/sql system

also you may want to look at siege, you can find it through freshmeat

if you need any help lemme know, i have lots of notes and some sample scripts.