How to generate random token on app/Mail VeryfyMail.php on Laravel?

working with Laravel 5.7 and I have VeryfyMail system. I need sending some random number to subject on email in app/Mail VeryfyMail.php file,

VeryfyMail.php

<?php

namespace App\Mail;

use Illuminate\Bus\Queueable;
use Illuminate\Mail\Mailable;
use Illuminate\Queue\SerializesModels;
use Illuminate\Contracts\Queue\ShouldQueue;

class VerifyMail extends Mailable
{
    use Queueable, SerializesModels;

    public $user;

    public function __construct($user)
    {
        $this->user = $user;
    }


    public function build()
    {
        //return $this->view('emails.verifyUser');
        return $this->subject('')->view('emails.verifyUser');
    }
}

how can I generate some ramdon number on above file subject?

via Chebli Mohamed