<?php
class foo {
    public function p() {} // everyone can use
    private function p2() {} // only class foo may use internally
    protected function p3() {} // only class foo and it's extender can use
}

class foo_ex extends foo { 
    function foo_ex() { 
        $this->p2(); 
    } 
}

$a = new foo();
$a->p3(); // fatal error

$b = new foo_ex(); // will fail with fatal error
?>