login code in php using mysql

<?php

include('conn.php');
session_start();
if(isset($_POST['submit']))
{
$query="select * from login where Username='".$_POST['username']."' and Password='".$_POST['Password']."'";
$result=mysql_query($query) or die(mysql_error());
$row=mysql_fetch_object($result);
$count=mysql_num_rows($result);
if($count>0)
    {
    $_SESSION['ID']=$row->ID;
    $_SESSION['Username']=$row->username;
   
    header('location:category_add.php');
    }
else
    {
echo $error_msg="invalid user name or password";
}

}
?>