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