Indent "child" comment under "parent" comment - indentation

Question

I'm creating a comment system where users can answer to an existent comment but i want to know how to indent those replies ("child" comment) under the existent comment ("parent" comment).
Here current code:
<h2>Fiche</h2>
<?php
$_SESSION['cf']['message'] = $_GET['message'];
$reponse = db_query('SELECT * FROM messages WHERE id = ?', array($_SESSION['cf']['message']));
$donneesmsg = $reponse->fetch()
?>
<h5><?php echo $donneesmsg['Pseudo']; ?></h5>
<p class="text-center text-align"><?php echo $donneesmsg['Message']; ?></p>
Repondre
<?php
echo '<h4>Commentaires</h4>';
echo '<hr>';
$reponse1 = db_query('SELECT * FROM comment WHERE messages_id = ?', array($donneesmsg['id']));
$donneescom1 = $reponse1->fetch();
$reponse = db_query('SELECT * FROM comment WHERE messages_id = ? ORDER BY id ASC', array($donneesmsg['id']));
while ($donneescom = $reponse->fetch()) {
if ($donneescom['commentaires_id'] == 0)
{
echo '<center>';
echo '' .$donneescom['id']. '<br>';
echo '' .$donneescom['pseudo']. '<br>';
echo '' .$donneescom['commentaire']. '<br>';
echo 'Repondre';
echo '<hr>';
echo '</center>';
}
if ($donneescom['commentaires_id'] != 0)
{
echo 'En reponse au com ' .$donneescom['commentaires_id']. '<br>';
echo '' .$donneescom['id']. '<br>';
echo '' .$donneescom['pseudo']. '<br>';
echo '' .$donneescom['commentaire']. '<br>';
echo 'Repondre';
echo '<hr>';
}
}
?>
<form id="formcom" method="post" action="index.php?page=addcomment">
Pseudo: <input type="text" placeholder="Pseudo" name="pseudo"> <br>
Mail: <input type="email" placeholder="Mail" name="mail"> <br>
Commentaire: <textarea placeholder="Commentaire" name="commentaire"></textarea> <br>
<input type="hidden" name="date" value="<?php echo time(); ?>"><br>
<input type="hidden" name="messages_id" value="<?php echo $donneesmsg['id']; ?>"><br>
<input type="text" name="commentaires_id" value="<?php echo $_GET['com']; ?>"><br>
<input type="hidden" name="id" value="<?php echo $donneesmsg['id']; ?>"><br>
<input type="submit" value="Envoyer">
</form>