In PHP 5 classes can also be defined as "abstract" classes, which must be extended before it can be instanciated

<?php
    abstract class foobar {

        abstract function myfunct();

    }

    class myfoobar extends foobar {

        function myfunct() { /* .... */ }
    }

    $a = new myfoobar;
    $a->myfunct();
?>