Your SQL prepared statement needs to be corrected as shudent said, but I think you also have to set the values with ->bind_param()
before calling ->execute()
(which does not take any parameters).
$stmt = $conn->prepare('INSERT INTO renter (email, msg_text) VALUES(?, ?)');
$stmt->bind_param("ss", $_POST['email'], $_POST['msg_text']);
$stmt->execute();
See documentation and example on the PHP official site : http://php.net/manual/fr/mysqli.prepare.php