When using AUTO_INCREMENT, you cannot know what value has been given to a column before a row is inserted. Instead, if you need to know it, you must ask MySQL afterward using the mysql_insert_id function. This need is common: for instance, when you process a purchase, you might insert a new customer into a Customers table and then refer to the newly created CustId when inserting a purchase into the purchase table.

require_once 'login.php';
$db_server = mysql_connect($db_hostname, $db_username, $db_password);
if (!$db_server) die("Unable to connect to MySQL: " . mysql_error());
or die("Unable to select database: " . mysql_error());
$query = "INSERT INTO cats VALUES(NULL, 'Lynx', 'Stumpy', 5)";
$result = mysql_query($query);
echo "The Insert ID was: " . mysql_insert_id();
if (!$result) die ("Database access failed: " . mysql_error());