On the PHPEasy.co.uk site there's a quick new tutorial posted about type hinting in PHP - how to use it and why you should use it in your code.
Type hinting allows you to tell a function which data type to accept for its arguments. When you call a function, if a type hint is present, the argument passed will be checked to make sure it is of the type specified. If a function has control over what it accepts as an argument, then it is more likely that the code within the function will have the data it needs to perform the task it is intended for.
He includes two code examples - one not using a type hinting on the parameters (assuming the correct arguments) and another forcing the input of the "PrintGuitar" method to be a "Guitar" object instance. You can find out more about type hinting in the PHP manual.