Login page problem

<?phprequire_once 'database.php';


session_start();


function signIn()
{
$sql = 'SELECT username, user_type FROM users WHERE username = "'.addslashes($_POST['username']).'" ';
$sql .= 'AND password = "'.addslashes(md5($_POST['password'])).'";';
dbConnect();
$result = runQuery($sql);


if (mysql_num_rows($result) != 0)
{
$data = mysql_fetch_assoc($result);
$_SESSION['username'] = stripslashes($data['username']);
$_SESSION['user_type'] = $data['user_type'];


if ($_SESSION['user_type'] == 'student')
{
header('Location: studentpage.php');
}
else if ($_SESSION['user_type'] == 'staff')
{
header('Location: adminpage.php');
}
}
else {
header('Location: index2.php?status=SIGN-IN-FAILED');


}
}


function signOut()
{
session_destroy();
header('Location: index2.php');
}


function isSessionValid()
{
if (!isset($_SESSION['username']) || empty($_SESSION['username'])) {
signOut();
}


return true;
}


if ($_POST['action'] == 'login')
{
signIn();
}


if ($_GET['logout'] == 'true')
{
signOut();
}
?>

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