facebook_deauth.php

        $facebook = new Facebook(array(
            'appId' => FACEBOOK_APP_ID,
            'secret' => FACEBOOK_APP_SECRET,
        ));

        // Get User ID
        $result = '';
        $user_id = $facebook->getUser();
        if ($user_id) {
            $access_token = $facebook->getAccessToken();
            $result = $facebook->api(array(
                'method' => 'auth.revokeAuthorization',
                'uid' => $user_id,
                'access_token' => $access_token
            ));
        }