php mysql update รบกวนถามหน่อย code php มันขึ้น error ครับ....................

<?php
session_start();
if(!isset($_SESSION['username']))
{
echo "<script>alert('!! Please Login !!');window.location='index.php';</script>";
}
include("connect_db.php");
$name = $_POST['name'];
$lastname = $_POST['lastname'];
$gender = $_POST['gender'];
$day = $_POST['day'];
$month = $_POST['month'];
$year = $_POST['year'];
$address = $_POST['address'];
$email = $_POST['email'];

if(empty($name)||empty($lastname))
{
echo "<script>alert('Please refill form !!!');history.back();</script>";
exit();
}

if(!checkdate($month,$day,$year))
{
echo "<script>alert('Please check Birthday!!!');history.back();</script>";
exit();
}
$birthday = "$year-$month-$day";
$sql= "update member set
name = '$name',
lastname = '$lastname',
gender = '$gender',
birthday = '$birthday',
address = '$address',
email = '$email',
where username = '{$_SESSION['username']}'";
mysql_query($sql) or die("error=$sql");
echo "<script>alert('++ Complete ++');window.location='main.php';</script>";
?>