On PHPMaster.com today there's a new tutorial posted about using type hinting in your PHP applications to restrict the values passed into your methods.
Since PHP 5 you can use type hinting to specify the expected data type of an argument in a function declaration. When you call the function, PHP will check whether or not the arguments are of the specified type. If not, the run-time will raise an error and execution will be halted.
Included in the post are code examples showing how to define custom types in a function definition and what happens if you pass the wrong type in. Also mentioned is one of the main limitations to hinting - the fact that it can't be used on default PHP variable types.