Peoples thoughts on processing tasks

Hi Guys, I am in the middle of a pretty involved php application. We have all the parts working in some shape or form and are ready to start redifining and cleaning up code, working things out and creating an order and 'template' in which to create our new projects. If anybody could offer there thoughts on the following, perhaps offering pros and cons (if it is not too much trouble). How do you go about processing functions and task. Do you keep them on one page. Then asign an action variable to the url, the script would then check to see what action is equal to and carry out its equivalent in an if statement. This process would also involve creating functions to do the things are done often. Or keep every task in a serperate php file. Having a file for each function that is done often. What about speed and reliability, does anybody know which is the faster method or if there is a better one? Anything else I should know about?? Thanks scott.