php 单例模式demo

//单例模式 demo
class singleton{
private static $instance;

private function __clone()
{ //防止clone
}

private function __construct()
{ //防止实例化
}

public static function GetInstance(){
if (!(self::$instance instanceof self)){
self::$instance = new self;
}
return self::$instance;
}

public function output($a){
echo $a;
}
}

$test_class = singleton::GetInstance();
$test_class->output('a');