<?php
if (isset($_POST['username'],$_POST['password']))
{
	$link = mysql_connect('localhost','root','');

	$login    = mysql_real_escape_string($_POST['username']);
	$password = mysql_real_escape_string($_POST['password']);
	/* возможно стоило заюзать mysqli */
	$result = mysql_query("SELECT COUNT(*) as cnt FROM  users WHERE username = '{$login}' AND password='{$password}'");
	$data = mysql_fetch_array($result);
	mysql_close($link);
}
$redirect = isset($_SERVER['HTTP_REFERER']) ? addslashes($_SERVER['HTTP_REFERER']) : '/'
header("Location:'{$redirect}'");
?>