a.php

<?php
class Hoge {
    public function foo() {
        $this->bar();
    }
    public function bar() {
        echo "Hoge::bar()\n";
    }
}

class Fuga extends Hoge {
    public function bar() {
        echo "Fuga::bar()\n";
    }
}

$fuga = new Fuga();
$fuga->foo(); // Fuga::bar()