<?php

namespace App\Http\Requests;

use CHHW\FormRequest\FormRequest;

class CreatePost extends FormRequest
{
    /**
     * Determine if the user is authorized to make this request.
     *
     * @return bool
     */
    public function authorize()
    {
        return true;
    }

    /**
     * Get the validation rules that apply to the request.
     *
     * @return array
     */
    public function rules()
    {
        return [
            "title" => "required|string",
        ];
    }

    public function withValidator($validator)
    {
        $validator->after(function ($validator) {
             if (true) {
                 $validator->errors()->add('field', 'Something is wrong with this field!');
             }
        });
    }
}

use App\Http\Requests\CreatePost;

class ExampleController extends Controller
{
    public function test(CreatePost $request)
    {
        dd($request->all());
        // ...
    }
}

use App\Rules\Uppercase;
use Illuminate\Http\Request;

class ExampleController extends Controller
{
    public function create(Request $request)
    {
        $request->validate(["author" => ["required", new Uppercase],]);
        // ...
    }
}

 <?php

namespace App\Rules;

use Illuminate\Contracts\Validation\Rule;

class Uppercase implements Rule
{
    /**
     * Determine if the validation rule passes.
     *
     * @param  string  $attribute
     * @param  mixed  $value
     * @return bool
     */
    public function passes($attribute, $value)
    {
        return strtoupper($value) === $value;
    }

    /**
     * Get the validation error message.
     *
     * @return string
     */
    public function message()
    {
        return 'The :attribute must be uppercase.';
    }
}

public function render($request, Throwable $exception)
{
    if($exception instanceof ValidationException){
        return response()->json(["message" => $exception->getMessage(), "details" => $exception->errors()]);
    }
    return parent::render($request, $exception);
}