James Fuller has a new post to his blog today about a way you can enforce contracts in your PHP using a combination of type hinting and value checking.

Design by contract is an important concept for controlling what type of input your methods or functions can receive. One of the most dangerous features of PHP is that functions will still execute even when they are missing required arguments, by emitting a warning instead of an error. In this post, I am going to walk through some of the solutions available to deal with this problem.

He shows how to alter a basic function to first use type hinting to catch when a variable is the wrong type (in this case checking for an array and stdClass) which causes a Fatal error and makes the function not execute. He includes sample code for the type/value checking option and also includes a suggestion of using PHPUnit's assertions as another option.

Finally, he introduces the ContractLib tool (from Stuart Herbert) that makes use of closures to enforce checks - his example checks to see if something is a string and that it's not empty.