อัพค่าขึ้น SQL ผ่าน PHP แล้ว Error แบบนี้ แก้ยังไงครับ

<?php
$tempSerialNumber = $_GET['sn'];
$tempLatitude = $_GET['l1'];
$tempLongitude = $_GET['l2'];

$servername = "localhost";
$username = "bornitex_uptoyou";
$password = "12345678";
$dbname = "bornitex_TMS";


$conn = new mysqli($servername, $username,$password, $dbname);
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
} 

$val1 = $_GET['tempSerialNumber'];
$val2 = $_GET['tempLatitude'];
$val3 = $_GET['tempLongitude'];

$sql = "INSERT INTO LogGPS (SerialNumber, Latitude, Longitude)
VALUES ($val1, $val2, $val3)";


if ($conn->query($sql) === TRUE) {echo "save OK";} else {
    echo "Error:" . $sql . "<br>" . $conn->error;}

$conn->close();
?>