Incidentally, abstract classes do not need to be base classes:
<?php
class Foo {
public function sneeze() { echo 'achoooo'; }
}
abstract class Bar extends Foo {
public abstract function hiccup();
}
class Baz extends Bar {
public function hiccup() { echo 'hiccup!'; }
}
$baz = new Baz();
$baz->sneeze();
$baz->hiccup();
?>