This is not going to help you with your scenario- how far does the packet travel? Will it stay inside the IP stack? Go past MS firewall? Maybe reach your router? Maybe reach your ISP? There could be a blocking system anywhere along the path that the client would not be able to find on his own. The only reliable thing is to set up a known server somewhere on the internet and ask it to send you a message. If you receive it then the port is unblocked.

In other words you'd need to
a) write a client that sends a request to your server containing your desired UDP port, ideally to some webhost that can be reached via TCP 80.
b) on the webhost when you get a request by a client, send a UDP package to the requested port
c) the client needs to bind to the UDP port and if it gets a response from the webhost it knows that there is no firewall in the way

Maybe use the script these guys have in place: http://www.utorrent.com/testport.php?port=6881

nmap: i suppose it reaches my ISP as it displays "interesting ports at brln-xxxxxx.xxxx.<my_isp>.de:"

however, the script solution seems to be much more elegant. where can i get the source?

[edit] as most online port checking solutions, this script did not see my port that i just opened for testing purposes. so far, nmap has been the only reliable one.

Try nmap not wiht localhost but with your local network address, or even with your Internet IP, perhaps this works...
Otherwise, use two computers

that's exactly what i wrote 2 posts above, and it works. but i'd prefer something smaller... really. for nmap, you need to install nmap and winpcap, both are no lightweight tools.

er... ok... I understand. You don't just want to test, you want to embed this in your server.
Well, do you really think, this is neccessary? I think it's the users task to ensure, the specific port is open.

That just tells you it could resolve the address using DNS - it's unrelated to actual route taken. You could do a tracert (or traceroute on unix) to see where the packet goes- but again, that is specific to your setup. With a different system the packet might stay local, not telling you anything about firewalls. You need an external server for port checking. Since you are familiar with PHP can't you write a script that sends a UDP packet to a specified IP and port ?