if ($stmt = mysqli_prepare($link, "SELECT id_mensaje, username, m.message, fecha_mensaje, avatar FROM mensaje_sbr m INNER JOIN usuarios_sbr u ON m.author = u.username WHERE id_mensaje_padre is NULL ORDER BY id_mensaje DESC"));
$stmt = mysqli_bind_param("isd", $author, $message, $fechReg);
$stmt = mysqli_execute();
$stmt = mysqli_bind_result($stmt);
$stmt = mysqli_fetch_array($stmt);
$stmt= mysqli_num_rows($stmt);
if($stmt= mysqli_num_rows==0){
echo "lo que sea";
}