I have this problem that is really causing me headeches whenever i'm designing my apps in php: I don't know if i should create separete files for each function(e.g.: functions for validating specific forms).
OK, one would possibily argue that this makes no sense because I would have to include each file separetly and this would result in a more slow application maybe?
But I still think it does make sense since for one pageload i doubt that other functions would be used by the script at all, so they must be loaded just for nothing? besides, i don't have to include each function-file manually if the system i design does this dinamically (parsing url vars or such) for me, that is, loading function(-files) exactly when needed. What do you think?
The overhead in file includes is minimal, you shouldn't have to worry about it really, considering caching and other things. Of It's more about how you can keep yourself organized and find your stuff quickly.
Honestly, I rarely use functions, I use classes. The rule is usually to have a class per file. But I also have a toolbox file that contains all my global functions.