Laravel 5 如何从多个下拉列表中选择数据，并将它们组合成laravel中的一个变量？

我有三个下拉列表，分别是出生日期数据的日期、月份和年份，以及数据库中的一列dob。我想将所选数据转换为出生日期格式，然后存储在数据库中。这是创建语法错误

我的控制器代码：

protected function create(array $data)
{ 
        'month'  => $data['month'],
        'day'  => $data['day'],
        'year'  => $data['year'],

     return User::create([
        'first_name' => $data['first_name'],
        'last_name' => $data['last_name'],
        'gender'    =>$data['gender'],
        'email' => $data['email'],
        'password' => bcrypt($data['password']),
        'phone' => $data['phone'],
        'dob'   => "month/day/year",
    ]);
}

$dob = $data['year'].'-'.$data['month'].'-'.$data['day'];

return User::create([
        'first_name' => $data['first_name'],
        'last_name' => $data['last_name'],
        'gender'    =>$data['gender'],
        'email' => $data['email'],
        'password' => bcrypt($data['password']),
        'phone' => $data['phone'],
        'dob'   => $dob,
    ]);