Session won't pick up one variable

[Terion Miller <webdev.terion@xxxxxxxxx>]

Think I'm setting this in the wrong place...someone help ...where do I set
the AdminID session

if (isset($_POST['UserName'])) {$UserName = $_POST['UserName'];} else
{$UserName = '';}
if (isset($_POST['Password'])) {$Password = $_POST['Password'];} else
{$Password = '';}

$msg = '';

if (!empty($UserName)) {

    $sql = "SELECT `AdminID`,`UserName` FROM `admin` WHERE
`UserName`='$UserName' and    `Password`='$Password'";
    $result = mysql_query ($sql);
    $row = mysql_fetch_object ($result);

    if (isset($_POST['AdminID'])) {$AdminID = $_POST['AdminID'];} else
{$AdminID= '';}

    If (mysql_num_rows($result) > 0) {
        $_SESSION['AdminLogin'] = true;
        $_SESSION['user']=$UserName;
        $_SESSION['AdminID']=$AdminID;

        header ('Location: Main.php');
        exit;
    } else {