something.php

<?php

class ParentClass {
    function __construct() {
        $this->self_define();
    }

    function self_define() {
        $this->foo = 'test';
    }
}

class ChildClass extends ParentClass {
    function __construct() {
        parent::__construct();
    }

    function self_define() {
        $this->foo = 'another';
    }
}

$c = new ChildClass();
echo $c->foo . "\n"; // prints "another"