Laravel [5.3]自定义验证获取规则中数组的键

<?php

namespace App\Rules;

use Illuminate\Contracts\Validation\Rule;

class CustomValidationRule implements Rule
{
    public function passes($attribute, $value)
    {
        // 检查数组中是否存在指定的键
        return array_key_exists('key', $value);
    }

    public function message()
    {
        return 'The :attribute must have a key.';
    }
}

$rules = [
    'field' => ['required', new CustomValidationRule],
];

$validator = Validator::make($data, $rules);