Store HTML Checkboxes (array) into a MySQL database

I want to store basic user input into a database via INSERT INTO. I've managed to store all input possible (text, tel...) but checkboxes is a problem as they are arrays! How can I store them?

vos_jeux.php (where checkboxes are displayed)

<form method="post" action="end.php">
  <p>
    Cochez les jeux désirés pour votre soirée:<br>

      <label for="blackjack"><img src="img/blackjack.jpg" alt=""></label>
      <INPUT id="blackjack" type="checkbox" name="game[]" value="Blackjack">    

      <label for="chuckaluck"><img src="img/chuckaluck.jpg" alt=""></label>
      <INPUT id="chuckaluck" type="checkbox" name="game[]" value="Chuck à Luck">

      <label for="roulette"><img src="img/roulette.jpg" alt=""></label>
      <INPUT id="roulette" type="checkbox" name="game[]" value="Roulette">

      <label for="stud"><img src="img/stud.jpg" alt=""></label>
      <INPUT id="stud" type="checkbox" name="game[]" value="Stud Poker">

      <label for="holdem"><img src="img/holdem.jpg" alt=""></label>
      <INPUT id="holdem" type="checkbox" name="game[]" value="Holdem Poker">

      <label for="boule"><img src="img/boule.jpg" alt=""></label>
      <INPUT id="boule" type="checkbox" name="game[]" value="La Boule">

    <input type="button" value="Retour en arrière" onClick="self.history.back();">
    <input type="submit" name="submit" value="Poursuivre">
  </p>
</form>

Look at my database: (see how "jeux" checkboxes don't appear)

end.php

<?php

include "connect.php";

$nom = $_POST['nom'];
$prenom = $_POST['prenom'];
$adresse = $_POST['adresse'];
$tel = $_POST['tel'];
$mail = $_POST['mail'];
$adresse2 = $_POST['adresse2'];
$date = $_POST['date'];
$nb_invite = $_POST['nb_invite'];
$evenement = $_POST['evenement'];
$game = implode(',', $_POST['game']);

$sql = "INSERT INTO donnees (nom, prenom, adresse, tel, email, adresse2, date, nb_invite, evenement, jeux)
VALUES ('$nom', '$prenom', '$adresse', '$tel', '$mail', '$adresse2', '$date', '$nb_invite', '$evenement', '$game')";

if (mysqli_query($conn, $sql)) {
    echo "";
} else {
    echo "Error: " . $sql . "<br>" . mysqli_error($conn);
}

mysqli_close($conn);
?>

1_ I tried the "implode" solution but it doesn't seem to work. What can I do? 2_ Echoing the $sql variable gives: INSERT INTO donnees (nom, prenom, adresse, tel, email, adresse2, date, nb_invite, evenement, jeux) VALUES ('foo', 'foo', 'foo', 'foo', 'foo', 'foo', 'foo', 'foo', 'foo', '') The results of the checkbox inputs is NULL '' ! Why?