As with PHP 4, classes that extend other classes should call the parent's constructor within their own constructor.

<?php
class old {
    function old() { $this->val = 123; }
}

class newer extends old {
    function __construct() {
        /* will work despite the older class using PHP 4 style constructor */
        parent::__construct();
    }
}

$a = new newer();
echo $a->val; // 123
?>