sample.php

<? php
 
public function validate($startDate, $endDate) {
    $rules = [
        'startDate' => 'date_format:Y-m-d',
        'endDate' => 'date_format:Y-m-d',
    ];
    $validator = Validator::make(['startDate' => $startDate, 'endDate' => $endDate], $rules);

    if ($validator->fails()) {
        return response()->json($validator->getErrors(), 422);
    }
}

public function getAuthenticatedUser() {
    return auth()->user();
}

public function formatDate($date) {
    $dateFormat = new Carbon($date);
    return $dateFormat->format('Y-m-d');
}

public function getDaysRange($min, $max) {
    $userId = $this->getAuthenticatedUser()->id;

    return Day::where('date', '>=', $min)
    ->where('date', '<=', $max)
    ->where('user_id', $userId)
    ->orderBy('date', 'ASC')
    ->get();
}

public function findMealById($dayId) {
    $userId = $this->getAuthenticatedUser()->id;

    Meal::where('day_id', $dayId)->where('user_id', $userId)->get();
}

public function getDayRange2(
      $startDate,
      $endDate
  ) {
      $this->validate($startDate, $endDate);
 
      $min = $this->date_format($startDate);
      $max = $this->date_format($endDate);
 
      $days = $this->getDaysRangeByUser($min, $max);
 
      foreach ($days as &$day) {
          $day->meals = $this->findMealById($day->id);
      }
 
      $max = $endDate->toImmutable();
      $min = $startDate->toImmutable();
 
      $transformedDays = [];
 
      while ($min <= $max) {
          foreach ($days as $day) {
              if ($day->date === $this->date_format($min)) {
                  $transformedDays[] = $day->toArray();
                  $min = $min->add('day', 1);
                  continue;
              }
          }
 
          $transformedDays[] = [
              'date'          => $this->date_format($min),
              'calorie_limit' => 1500,
              'total_calories' => 0,
              'meals' => []
          ];
 
          $min = $min->add('day', 1);
      }
 
      return response()->json($transformedDays);
  }

For validation I Will use a custom validation request instead of doing it on the same file as the function getDayRange2, so for that I Will create a new request using Laravel

use Illuminate\Foundation\Http\FormRequest;
use Illuminate\Http\JsonResponse;

class CustomRequest extends FormRequest
{

    public function rules()
    {
        $rules = [
            'startDate' => 'date_format:Y-m-d',
            'endDate' => 'date_format:Y-m-d',
        ];

        return $rules;
    }

    public function response(array $errors)
    {
        return new JsonResponse(['error' => $errors], 422);
    }
}

So as you can see I can override the public function response(array $errors) method and return a modified response without Validator explicitly.

And also instead of returning transformedDays I will use Laravel collection Resources, so for that, I will create a new class Day:

namespace App\Http\Resources;

use Illuminate\Http\Resources\Json\JsonResource;

class Day extends JsonResource
{
    /**
     * Transform the resource into an array.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return array
     */
    public function toArray($request)
    {
        return [
            'id' => $this->id,
            'calorie_limit' => $this->calorie_limit,
            'email' => $this->email,
            'total_calories' => $this->total_calories,
            'meals' => [] // here I will use an relationship between Meal and Day -> PostResource::collection($this->meals)
        ];
    }
}