Apache and PHP are already installed.
PHP is turned off by default. To turn it on, check the following link:
http://clickontyler.com/blog/2007/10/how-to-enable-php5-in-mac-os-x-leopard/
To turn on apache, go to System Prefrences, Sharing, check the Web Sharing box and you're all set.
To install mysql server, check the following link (assumes that you have the Developer tools and you compile from source):
http://hivelogic.com/articles/installing-mysql-on-mac-os-x/
You should be good to go. I just set the above up about a week ago following those steps.