Membaca nilai kotak centang menggunakan fungsi implode

coba ini

if(isset($_POST['Submit'])) {
    echo "<pre>"; print_r($_POST);
    $checked = implode(',', $_POST['checkbox']); 
    echo $checked;
}

inilah hal-hal persis yang saya masukkan ke dalam test.php saya

<?php
if(isset($_POST['Submit'])) {
    echo "<pre>";
    $checked = implode(',', $_POST['checkbox']);
    echo $checked;
}

?>
<form action="test.php" method="post">
<input type="checkbox" name="checkbox[]" value="hostess_name">
<input type="checkbox" name="checkbox[]" value="hostess_familyname_en">
<input type="checkbox" name="checkbox[]" value="hostess_id">
<input type="checkbox" name="checkbox[]" value="hostess_firstname_en">
<br>
<br>
<input type="submit" name="Submit" value="Submit">
</form>