How to calculate the average of total ratings of 5 stars?

I’m trying to get the average rating of total star rating for one restaurant. The customers will rate this restaurant. I made a pivot table for storing customers ratings.

The average rating =5

--------
place_ratings
---------------
user_id   place_id rating
   1          1      4
   2          1      3
   5          1      5

What is the formula or how can I calculate that in php laravel ?
This is my code:

public function placeRating($id)
    {

        $rates = PlaceRating::where('place_id',$id)->select('rating')->get()->toArray();

           $rateArray =[];
           foreach ($rates as $rate)
           {
               $rateArray[]= $rate['rating'];
           }

            $sum = array_sum($rateArray);
            $result = $sum/5;

            return response()->json(['rating'=>$result],200);
    }

Answer

Don’t take all place_ratings records out, if there are thousands+ user rate the restaurant, it will slow your API.

Try to use this query:

PlaceRating::where('place_id',$id)->selectRaw('SUM(rating)/COUNT(user_id) AS avg_rating')->first()->avg_rating;