Yesterday I moved my nagios-server to a newer hardware and started to do all from scratch (good learning lesson on mistakes).
One tool for nagios trap-administration is a project called nagtrap (Nagtrap).
The installation of the tool is quite easy. Just extract the tar file and copy it to the webservers root directory. What is not easy, or often overread is the fact that this tool needs a php variable enabled.
short_open_tag = On
By default on php5 this one is disabled. By enabling it, the webpage of nagtrap will be shown instead of the sourcecode.