How about the PHP Manual at php.net...
You can also go to Hotscripts.com to download a premade script. I don't know if they have text based RPG scripts, but you should look.
As far as "Where to start", you need to sit down and storyboard what you want it to do, and build your code around your storyboard, but if you don't know how to program in PHP, then you need to sit down with the manual and learn the basics.
Or, if you are nice, you may find someone who is willing to write it for you... Good Luck either way.
