If you are using linux or similar, just type ifconfig at the command line, you are looking for an address associated with a block of text starting PPP some number.
In windows 98 run winipcfg from the run box in the start menu, that will tell you what's configured.
There's something similar for NT ipconfig but it has to be run from a command line I think.
It's probably the same for Win ME and 2000 but, I dunno for sure.
If you want to run these from within a PHP script you might try using exec but I've never tried it.
Hope this helps
Paul