我將在流明中使用 gmail smtp 發送電子郵件，一切正常，但一個變量始終未定義，請讓我知道我錯在哪里這是我的代碼<?phpnamespace App\Services;use Illuminate\Support\Facades\Mail;class MailService{    public static function send($mail_to = '', $title = '', $content = '') {        Mail::send('mail', ['title' => $title, 'content' => $content], function ($message) {            $message->from('[email protected]', 'Test Mail');            $message->to($mail_to);        });    }}這是控制器public function register(Request $request){    $rules = [        'name'          => 'required',        'email'         => 'required|email|unique:users',        'password'      => 'required|min:5',        'phone'         => 'required|numeric|min:10',        'business_name' => 'required|unique:users',        'business_type' => 'required'    ];    $this->validate($request, $rules);    $data = $request->all();    $hashPassword = Hash::make($data['password']);    $data['password'] = $hashPassword;    $data['is_activated'] = 'false';    $pin    = mt_rand(1000, 9999);     $token  = hash("sha256", $pin);    $data['token']  = $token;    $data['otp']    = $pin;    $user = User::create($data);    if ($user) {        MailService::send($request->input('email'), 'OTP', $pin);        return response()->json(['response' => true, 'message' => 'User registered Successfully', 'token' => $token], 201);    } else {        return response()->json(['response' => false, 'message' => ' Please check your credentials, Try again'], 400);    }}這是錯誤{消息：“未定義變量：mail_to”，異常：“ErrorException”，文件：“D:\xampp\htdocs\api\app\Services\MailService.php”，行：12，跟蹤：Array(28)}異常：“錯誤異?！蔽募骸癉:\xampp\htdocs\api\app\Services\MailService.php” 行：12 消息：“未定義變量：mail_to”