Class type hints

Although PHP remains a loosely typed language, which means you do not need to specify what types a variable is when it is declared or passed to a function, PHP 5 introduced class type hints, which allow you to specify what kind of class should be passed into a function. These are not required, and are also not checked until the script is actually run, so they aren't strict by any means.

Furthermore, they only work for classes right now - you can't specify, for example, that a parameter should be an integer or a string. Having said that, I suspect future versions may introduce the ability to request that arrays are passed in - that's pure speculation, though! (Edit: this was subsequently added in PHP 5.1 - hurrah!)

Here is an example of a type hint in action:

<?php
    class dog {
        public function do_drool() {
            echo "Sluuuuurp\n";
        }
    }

    class cat { }

    function drool(dog $some_dog) {
        $some_dog->do_drool();
    }

    $poppy = new cat();
    drool($poppy);
?>

Note that the drool() function will accept one parameter, $some_dog, but that the parameter name is preceded by the class hint - I have specified that it should only accept a parameter of type "dog". In the example, I have made $poppy a cat object, and so running that will give the following output:

Note that providing a class hint for a class type that does not exist will cause a fatal error. As you can see, class hints are essentially a way for you to skip having to use the instanceof keyword again and again to verify your functions have received the right kind of objects - using a class hint is essentially implicitly using instanceof, without the extra code. You should make use of class hints regularly as they are a very simple way to make your code regulate itself, and helps solve bugs faster.

Copyright ©2015 Paul Hudson. Follow me: @twostraws.