Best method for threading?

I'm working on a web spider application where the server has considerable latency in serving the information I require, but simultaneous requests do not have a significant performance hit. I have a nice little class that handles all the sessions, cookies, etc perfectly. What's the best way to basically hand off a bunch of threads to access this information without hanging up the execution of my script? I plan to handle inter-thread coordination via MySQL and code in the main script. I've done threads before like this and have had great luck, but all my solutions have been hack-ish at best. What are the cleanest solution for this? What do you all do to handle situations like this? I will be running PHP 4.3.1.0 under Linux and Mac OS X, depending on location. Thoughts anybody?