To get to a command line, open up Terminal, located in the Utilities folder. Download Marc Liyanage's great
PHP and
MySQL binaries to get them running on the puter.
For a text editor, you absolutely must use
BBEdit
For mutliple users, just start adding new users as administrator using system preferences... each one is protected with proper unix privacy settings. If you want extra security, you can have your home folder encrypted (open system prefs and type encrypt in the search box).
Get a hold of
Transmit - the best FTP client for the mac.
You'll also need
Video Lan Client
There's a tonne of shit to play around with - just spend a week fully getting to know your system, check out versiontracker.com for all mac software.
Start learning all the keyboard shortcuts - your life will become 1000x easier
And welcome to the Mac world!