"charset=UTF-8" sending mail in php - php

Question

My teacher asked us to be able to send email via our website. As I speak french, I tried using charset=UTF-8 in the header. It works well for mail address, headers, name with special char but not for the message of the mail itself...
I always receive message like =?UTF-8?B?w6nDqSDDqMOoIMOgw6Agw6fDpw==?= .
Here is my php codes :
function contenuContactHTML(){
echo ' <div id="contenu">
<h1> Formulaire de Contact </h1>
<form action="" method="post">
Votre Nom: <input type="text" name="prenomFormulaire"><br>
Votre E-mail: <input type="text" name="emailFormulaire"><br>
Sujet du message : <input type="text" name="sujetMailFormulaire"><br>
Votre Message: <textarea cols="40" rows="6" name="messageFormulaire" maxlength="4000" placeholder="message limité à 4000 cractères" required></textarea><br />
<input type="submit" name="envoieMailContact" value="Envoyer E-Mail">
</form>
</div><!-- #contenu -->
</div><!-- #centre --> ';
envoieMail();
}
function envoieMail(){
if(isset($_POST['envoieMailContact'])){
$a = <i> My email adress</i>;
$sujet = "=?UTF-8?B?".base64_encode($_POST['sujetMailFormulaire'])."?=";
$de = "=?UTF-8?B?".base64_encode($_POST['emailFormulaire'])."?=";
$messFormulaire = $_POST['messageFormulaire'];
$message = "=?UTF-8?B?" . base64_encode($messFormulaire) . "?=";
$entete = "From: ". $de ."\r\n"
."MIME-Version: 1.0" . "\r\n"
.'Content-type: text/html; charset=UTF-8' .'\r\n';
mail($a, $sujet, $message, $entete);
echo "<script language='JavaScript'>alert('Message envoyé !')</script>";
}
}
Here is an example of the mail i receive :
email adresse : mické#hotmail.com
sujet (header) : on é pas des pigeaôns
content of the mail (message) :
=?UTF-8?B?YXplcnR5dWlvcF4kDQpxc2RmZ2hqa2xtw7nCtQ0Kd3hjdmJuLDs6DQrCsibDqSInKMKnw6ghw6fDoCkt?=