API to get all the reviews and rating from Google for business

You should need to use the Google My Business API if you want to get all the reviews.

It's another different API, and you should request from Google the property of the place.

https://developers.google.com/my-business/content/review-data#list_all_reviews

Other simple method to get the 5 last rewiews with the place_id of the location (you can find it here : https://developers.google.com/places/place-id)

$url = "https://maps.googleapis.com/maps/api/place/details/json?key=Yourkey&placeid=YourplaceID";
$ch = curl_init();
curl_setopt ($ch, CURLOPT_URL, $url);
curl_setopt ($ch, CURLOPT_RETURNTRANSFER, 1);
$result = curl_exec ($ch);
$res        = json_decode($result,true);
$reviews    = $res['result']['reviews'];