Laravel8 有効な日付バリデーション

<?php

namespace App\Http\Requests\User;

use Illuminate\Foundation\Http\FormRequest;
use App\Rules\User\BirthdayRule;　// BirthdayRuleを使うことを宣言

class MemberRequest extends FormRequest
{
    private $birth_y;
    private $birth_m;
    private $birth_d;

    public function authorize()
    {
        return true;
    }

    public function all($keys = null)
    {
        $results = parent::all($keys);

        // '年','月','日'をプライベート変数に退避
        $this->birth_y = $results['birth_y'] ?? null;
        $this->birth_m = $results['birth_m'] ?? null;
        $this->birth_d = $results['birth_d'] ?? null;

        return $results;
    }

    public function rules()
    {
        return [
            'birth_y' => [
                'required',
                'max:4',
                new BirthdayRule(
                    $this->birth_y,
                    $this->birth_m,
                    $this->birth_d,
                )
            ],
            'birth_m' => 'required|alpha_num|max:2',
            'birth_d' => 'required|alpha_num|max:2',
        ];
    }

    public function attributes(){
        return [
            'birth_y' => '生年月日（年）',
            'birth_m' => '生年月日（月）',
            'birth_d' => '生年月日（日）',
        ];
    }
}

<?php

namespace App\Rules\User;

use Illuminate\Contracts\Validation\Rule;

class BirthdayRule implements Rule
{
    private $birth_y;
    private $birth_m;
    private $birth_d;

    public function __construct($birth_y, $birth_m, $birth_d)
    {
        $this->birth_y = $birth_y;
        $this->birth_m = $birth_m;
        $this->birth_d = $birth_d;
    }

    public function passes($attribute, $value)
    {
        return checkdate(
            (int)$this->birth_m,
            (int)$this->birth_d,
            (int)$this->birth_y
        );
    }

    public function message()
    {
        return '誕生日は、有効な日付を指定して下さい。';
    }
}