session_start();
if (!isset($_SESSION["loggein"]) || $_SESSION["loggein"] == false) {
  echo "done";
  }
 ?>
 <h2>you have logged in</h2>
 <form class="" action="Insert.php" method="post">
   Day: <input type="text" name="day" value="">
   message: <input type="textbox" name="message" value="" rows="10">
   <input type="submit" name="" value="Send">
 </form>

insert.php

$day = mysqli_real_escape_string($conn, $_REQUEST['day']);
$message = mysqli_real_escape_string($conn, $_REQUEST['message']);
$username = mysqli_real_escape_string($conn, $_REQUEST[get_current_user($username)]);

$stmt = "INSERT INTO userInfo (day, message, username) VALUES('$day', '$message', '$username')";
if(empty($day) || empty($message)){
  echo "fyll i tbx revyn";
} else {
  if (mysqli_query($conn, $stmt)) {
    echo "added";
    header('Location: success.php', true, 303);
        exit;
  }else {
    echo "Error: " .mysqli_error($conn);
  }
}