A.phpclass A { protected $message; function __construct() { global $app; $this->message = $app->message; } public function aaa() { return $this->message['userError_2']; } public function ccc() { return B::bbb() } public function fff() { return $this->message['userError']; }}B.phpclass B { protected $message; function __construct() { global $app; $this->message = $app->message; } public function bbb() { return $this->message['userError']; }}在执行 B::ccc() 出现错误Message: Using $this when not in object context但执行 A::fff() 是可以成功拿值的这是什么意思？