Domain & Sub-domain Handling with PHP

Hi, I've done loads and loads with PHP and MySQL and stuff like that... but never had to really deal with actually creating new domains and subdomains with PHP. Especially with subdomains, I've seen lots of sites create subdomains automatically. What is the "best" technique for doing all this (if there is one)? I'm sure I could hack something together (I always have) but I'd really like the "best" way - cleanest, fastest, whatever the technique may be. Thanks, Galen