Ya didnt read the column did you. That shows you in detail exactly how to get Flash and PHP to communicate. If you want actual code, go get a php script that displays a page hit every so often, and hack it following the tutorial to make it show up in flash. As far as the number of users online. You can guess, but there is know way to know for 100% certainty how many are online, because http is a connectionless protocol.
You can guess, but not be for shure for shure.
Hope that Helps!