Mr Happy, i dont really think using integers is a great advantage apart from sanitization i.e typecasting your can lose yourself in your code if your getting into the 100+ actions, this i would keep with reply,edit,delete what i tend to do is to develop a MVC Framework and create a sort of REST system so:

Instead of topic.php?mode=reply&id=22 i would do

/topic/reply/22/

this way you only have to sanitize the topic/reply values so that they only allowed to be called IF the function exists within the topic class, this gives better security then within the reply method i just intergize the 3rd param.

This gives alot more structure and security to your code.