Classes and methods can be specified as final that will disallow classes to be extended
and methods to be overloaded.
<?php
final class foo { function do_something() {} }
class bar extends foo {} // won't work, fatal error
class baz { final function foo() {} function bar() {} }
class foo2 extends baz { function bar() {} } // will work
class foo3 extends baz { function foo() {} } // will not work
?>