<?php
class NullSessionHandler implements SessionHandlerInterface
{
    public function open($savePath, $sessionName)
    {
        return true;
    }
    public function close()
    {
        return true;
    }
    public function read($sessionId)
    {
        return '';
    }
    public function write($sessionId, $data)
    {
        return true;
    }
    public function destroy($sessionId)
    {
        return true;
    }
    public function gc($lifetime)
    {
        return 1;
    }
}

$handler = new NullSessionHandler();
session_set_save_handler($handler, true);
session_start();
$result = session_gc();
var_dump($result);