fb_long_access_token.php

<?php

$appId = 'aaaaaaaaaaaa';
$appSecret = 'bbbbbbbbbbbb';

// init facebook
$facebook = new Facebook( array( 'appId' => $appId, 'secret' => $appSecret ) );

// get user_id, 0 if not logged in
$user = $facebook->getUser();

// check if user is logged in
if ( $user ) {
    // get long-lived access token
    $facebook->setExtendedAccessToken();
    $access_token = $facebook->getAccessToken();
}