How to submit a checkbox in a feedback form?

Question

PHP

Hello, I immediately say that I don’t know PHP at all.
You need to make a feedback form, where there are several checkboxes.

<?php

$recepient = "[email protected]";
$sitename = "Текст";

$name = trim($_POST["name"]);
$phone = trim($_POST["phone"]);


$pagetitle = "Новая заявка с сайта \"$sitename\"";
$message = "Имя: $name \nТелефон: $phone \n$_POST[checkbox] ";
mail($recepient, $pagetitle, $message, "Content-type: text/plain; charset=\"utf-8\"\n From: $recepient");

<input type="checkbox" class="checkbox" name="checkbox[]"  value="rassupchataia" id="">Рассыпчатая пудра<br />
                  <input type="checkbox" class="checkbox" name="checkbox[]"  value="presovanaia" id="">Прессованная пудра<br />
                  <input type="checkbox" class="checkbox" name="checkbox[]"  value="1" id="">Набор №1<br />
                  <input type="checkbox" class="checkbox" name="checkbox[]"  value="2" id="">Набор №2<br />
                  <input type="checkbox" class="checkbox" name="checkbox[]"  value="3" id="">Набор №3<br />
                  <input type="checkbox" class="checkbox" name="checkbox[]"  value="4" id="">Набор №4

The message comes, but instead of checkboxes array is written there. Help sorting through the array of checkboxes please.