PHP单例模式实例,连接数据库对类的引用

<?php
//单例模式连接数据库
class pzhang{
    static private $instance;
    private static $config;
    private $dbase = array(
        'host' => 'localhost',
        'username' =>'root',
        'password'=>'root',
        'dbname' =>'jmyp'
    );
    private function __construct(){
    }
    static public function getInstance(){
         if(!self::$instance instanceof self)
             self::$instance  = new self();
         return self::$instance;
    }
    public function conn(){
        $mysql_db = $this->dbase['dbname'];
        self::$config = new mysqli($this->dbase['host'],$this->dbase['username'],$this->dbase['password']);
        self::$config->query('set name utf8');
        self::$config -> select_db($mysql_db);
        $sql = "select * from admin";
        $row = self::$config->query($sql);
        $data = [];
        while($tmp = $row->fetch_assoc()){
            $data[] = $tmp;
        }
        echo "<pre>";
        print_r($data);
        echo "</pre>";
        return self::$config;
    }
}
$obj = pzhang::getInstance();
$obj->conn();

//单例模式对类的引用
class zhangp{
    public function system(){
        echo "learning more";
    }
}

class singleCase{
    static private $instance;
    private $avg;
    private function __construct($config){
        self::$instance = new $config;
    }
    static public function getInstance($avg){
        if(!self::$instance instanceof self)
             new self($avg);
        return self::$instance;
    }
}
$obj = singleCase::getInstance('zhangp');
$obj->system();
?>