单态设计模式

class DB{

private static $obj=null;

private function __construct(){

echo "链接数据库成功
";

}

static function getInstance(){

if(is_null(self::$obj))

self::$obj=new self();

return self::$obj;

}

function query($sql){

echo $sql;

}

}

$db=DB::getInstance();

$db->query("select * from user");

?>