php Session issues

<? // THIS USES THE SHORT-OPEN TAG.  SUGGEST YOU CONVERT TO THE FULL TAG LIKE <?php

// THIS NEEDS TO BE THE FIRST LINE OF EVERY SCRIPT
error_reporting(E_ALL);

// set up database
$Host = "db380492857.db.1and1.com";
$User = "dbo380492857";
$Password = "abcxyz";
$DBName = "db380492857";

// THE FOLLOWING INSTRUCTION CAN FAIL, AND MUST BE TESTED FOR SUCCESS
$Link = mysql_connect ($Host, $User, $Password);

// THE FIELDS HERE NEED TO BE ESCAPED
$qry = "SELECT * from admin where code = '" . $_POST['uc'] . "' and password = '" . $_POST['pwd'] . "'";

// THIS FUNCTION IS DEPRECATED - CHANGE IT. http://php.net/manual/en/function.mysql-db-query.php
$res = mysql_db_query ($DBName, $qry, $Link);

// HOW DO YOU KNOW WHAT VALUE IN IN $res?  YOU HAVE TO TEST IT BEFORE YOU TRUST IT IN ANOTHER FUNCTION
$n = mysql_num_rows($res);

// WHAT WOULD YOU DO IF $n == 3?  MAYBE YOU NEED A LIMIT IN THE QUERY
if ($n == 0) {
	header("Location: login.php?bad=1");
	exit;
}

// THE SESSION IS NOT STARTED UNLESS THE LOGIN IS SUCCESSFUL?  THAT IS A RECIPE FOR CONFUSION
session_start();
$_SESSION['vavusr'] = $_POST['pwd'];
$_SESSION['user'] = $_POST['uc'];
$_SESSION['alast_used'] = time();	
header("Location: admin_menu.php");
exit;

// THE ZEND CODING STANDARD RECOMMENDS ELIMINATING THE CLOSING PHP TAG. OMIT IT.
?>

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