PHP单例模式

<?php
/**
*
*/
class DB
{
public $link;

private static $db;//看当前这个类有没有被实例化

private function __construct($host,$user,$pwd,$dbname){
$this->link = mysqli_connect($host,$user,$pwd,$dbname);
}

private function __clone(){}

//单例模式的核心思想：自己调用自己
public static function connect_mysql($host,$user,$pwd,$dbname){
if(!(self::$db instanceof self)){
self::$db = new self($host,$user,$pwd,$dbname);
}
return self::$db;
}

 

}

$obj = DB::connect_mysql('127.0.0.1','root','root','1812a');
print_r($obj);