Specification
----------
The testfeed.xml file is a sample output feed we receive from one of our vendors.  Write a code that will read/parse the xml file and output the list of userids who are currently online in the system.

XML attributes:
presence type 
	'd' for delta - the list of users who either logged off or logged on since last check/query
	'f' for full, meaning a list of all users who are currently logged on
on - list of online users
off - list of offline users
u - userid

Constraints: you can only use PHP4 xml functions (can't use SimpleXML, etc as they're PHP5)
Challenge: Consider how you get full lists when you get 'f' and changes if 'd'.  How would you sync your records to ensure that you remove those that log off and insert those who recently log in?
----------


Usage
----------
php runOnlineUpdate.php testfeed_d.xml
php runOnlineUpdate.php testfeed_f.xml

All errors and warnings get written to the error.log but i'm .gitignore'ing that so it's not checked in.
Right now DEBUG is on in online_update_feed.php, so all the debug output is going to be displayed.
If you set DEBUG to false, only the online user_id's will get outputed except when their is a failure to write to the error.log file.