logout.php

<?php
if (isset($_GET['logout'])) {
session_start(); # NOTE THE SESSION START
$_SESSION = array();
session_unset();
session_destroy();

// echo "Logged Out!";
// Note: Putting echo "Logged Out!" before sending the header could result in a "Headers already sent" warning and won't redirect your page to the login page - pointed out by @Treur - I didn't spot that one.. Thanks...
header("Location:index.php");
exit(); # NOTE THE EXIT
}