Error not shown developing own plugin

: Function ereg_replace() is deprecated in /mozdev/sandbox/php/include/globals.h on line 95.

I want to develope a Plugin for my own site. It doesn't work at the moment, so I search for the errors.
Now my Problem: There were no errors shown, but all error handels are on. Is there some other way to show the errors? I really need it, or I despair...
Thanks very much for your help :)
Nils