xyzshop/core/framework/libraries/CoMysqliPool.php

<?php

class CoMysqliPool extends CoRefPool
{
    const max_clients = 100;

    private static $stInstance = null;

    public static function instance()
    {
        if (self::$stInstance == null) {
            self::$stInstance = new CoMysqliPool();
        }
        return self::$stInstance;
    }

    private function __construct()
    {
        parent::__construct(self::max_clients);
    }

    public function create_client()
    {
        return ['master' => null, 'slave' => null];
    }
}