// Identification du client avec HTTP Basic Authentication :
$data = array(
    'grant_type'     => 'refresh_token',
    'refresh_token'  => $refresh_token,
); 
$h = curl_init($token_endpoint);
curl_setopt($h, CURLOPT_RETURNTRANSFER, true);
curl_setopt($h, CURLOPT_TIMEOUT, 10);
curl_setopt($h, CURLOPT_POST, true);
curl_setopt($h, CURLOPT_USERPWD, "{$client_id}:{$client_secret}");
curl_setopt($h, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($h, CURLOPT_HTTPHEADER, array('Content-Type: application/x-www-form-urlencoded'));
curl_setopt($h, CURLOPT_POSTFIELDS, http_build_query($data));

$res = curl_exec($h);

if ( is_array(json_decode($res, true) ) ) {

    curl_close($h);
    $res = json_decode($res, true);

}