Raising a custom error -- how?

Hiya, I want to be able to raise an error in some code I'm creating for reuse, most parts of this code deals with errors and just uses defaults as appropriate, but I need to be able to raise an error that will stop the PHP script interpreter and throw an error back to the developer. I've tried searching the manual for "user defined error", "throw error", "raise error" etc, but I guess it's just one of those functions that it's hard to find without already knowing where to find it. Any help would be appreciated!, Dw.