这部分代码为什么可以作为 mail：：send 的一个参数呢，不是数组也不是变量？

匿名函数（Anonymous functions），也叫闭包函数（closures），允许 临时创建一个没有指定名称的函数。最经常用作回调函数 callable参数的值。当然，也有其它应用的情况。

闭包函数也可以作为变量的值来使用。PHP 会自动把此种表达式转换成内置类 Closure 的对象实例。把一个 closure 对象赋值给一个变量的方式与普通变量赋值的语法是一样的，最后也要加上分号：

vendor/laravel/framework/src/Illuminate/Mail/Mailer.php

/**
     * Send a new message using a view.
     *
     * @param  string|array|\Illuminate\Contracts\Mail\Mailable  $view
     * @param  array  $data
     * @param  \Closure|string|null  $callback
     * @return void
     */
    public function send($view, array $data = [], $callback = null)
    {
        if ($view instanceof MailableContract) {
            return $this->sendMailable($view);
        }

        // First we need to parse the view, which could either be a string or an array
        // containing both an HTML and plain text versions of the view which should
        // be used when sending an e-mail. We will extract both of them out here.
        [$view, $plain, $raw] = $this->parseView($view);

        $data['message'] = $message = $this->createMessage();

        // Once we have retrieved the view content for the e-mail we will set the body
        // of this message using the HTML type, which will provide a simple wrapper
        // to creating view based emails that are able to receive arrays of data.
        call_user_func($callback, $message);

        $this->addContent($message, $view, $plain, $raw, $data);

        // If a global "to" address has been set, we will set that address on the mail
        // message. This is primarily useful during local development in which each
        // message should be delivered into a single mail address for inspection.
        if (isset($this->to['address'])) {
            $this->setGlobalToAndRemoveCcAndBcc($message);
        }

        // Next we will determine if the message should be sent. We give the developer
        // one final chance to stop this message and then we will send it to all of
        // its recipients. We will then fire the sent event for the sent message.
        $swiftMessage = $message->getSwiftMessage();

        if ($this->shouldSendMessage($swiftMessage, $data)) {
            $this->sendSwiftMessage($swiftMessage);

            $this->dispatchSentEvent($message, $data);
        }
    }