$stmt->bindParam(":gr_news", $t3); $t3 = $_POST['$t3'];Par ceci
$stmt->bindParam(":gr_news", $_POST['$t3']);
$stmt->execute(array( ":id" => $id, ":titre" => $t1, ":pet_news" => $t2, ":gr_news" => $t3, ":jour"=> $j, ":date" => $date, ":heure" => $heure ));