A few days ago, I started working with Simple MVC Framework, which is a lightweight, clean and yet very powerful PHP Framework.
Thereby I discovered a nice little helper to validate input data. It’s called GUMP and it seems to be quiet popular among some developers. GUMP is a standalone PHP data validation and filtering class that makes validating any data easy and painless without the reliance on a framework.
There is only one disadvantage I came across with. Currently, there is no implementation of localized error output, so you need to override the output method manually (a), create a hook that overrides the readable output (b) or use the error method to design an individual output (c). There are several ways creating localized output messages, choosing the most appropriate depends on which framework or code environment you use.
Nevertheless GUMP is a great class when it comes to data validation in all varieties. You can find the documentation here. Additionally, you might also follow along on the project board.