Assert

Assert is a PHP library that provides an alternative to PHP's  that allows for a simple and reliable way to check preconditions and postconditions in PHP code. It was proposed as a MediaWiki RFC, but is completely generic and can be used by any PHP program or library.

Usage
The Assert class provides several static methods for checking various kinds of assertions. The most common kind is to check the type of a parameter, typically in a constructor or a setter method:

Checking parameters, or other assertions such as pre- or postconditions, is not recommended for performance critical regions of the code, since evaluating expressions and calling the assertion functions costs time.