Asynchronous MySQL code

MysqlWrapper.php

<?php

namespace NiisanTokyo\As2sm;

use Evenement\EventEmitter;
use React\EventLoop\LoopInterface;

/**
 * The asynchronous wrapper for MySQL
 */
class MysqlWrapper extends EventEmitter
{
    
    const INTERVAL = 0.001;// The periodic timer cycle
    
    private static $_loop = null;
    
    /**
     * Setting the default loop.
     * 
     * if you want to use this class without regard to loop.
     * 
     * @param LoopInterface $loop loop
     */
    public static function setDefaultLoop(LoopInterface $loop)
    {
        self::$_loop = $loop;
        
    }
    
    private $connection = null;
    
    private $loop = null;
    
    private $timer = null;
    
    public function __construct($config, LoopInterface $loop = null)
    {
        $this->connection = new \mysqli(
            $config['host'],
            $config['user'],
            $config['pass'],
            $config['db']
        );
        
        $this->loop = ($loop === null) ? self::$_loop : $loop;
        
    }
    
    /**
     * Create asynchronous query.
     * 
     * @param type $sql
     * @param type $callback
     * @return type
     */
    public function query($sql, $callback = null)
    {
        $this->connection->query($sql, MYSQLI_ASYNC);
        $that = $this;
        
        $this->timer = $this->loop->addPeriodicTimer(self::INTERVAL, function () use ($that) {
            $that->checkPolling();
        });
        
        if ($callback === null) {
            return;
        }
        
        $this->on('success', $callback);
        
    }
    
    /**
     * Disconect from the mysql stream
     */
    public function disconnect()
    {
        $this->connection->kill($this->connection->thread_id);
        
    }
    
    /**
     * Checking the connection status;
     * 
     * @return type
     */
    private function checkPolling()
    {
        $read = $error = $reject = [$this->connection];
        if (! \mysqli::poll($read, $error, $reject, 0, 1)) {
            return;
        }
        
        if ($res = $this->connection->reap_async_query()) {
            $this->emit('success', [$res]);
        } else {
            $this->emit('error');
        }
        
        $this->loop->cancelTimer($this->timer);
        
    }
}