<?
$user = $_POST['username'];
$password = $_POST['password'];

$user = stripslashes($user);
$password = stripslashes(md5($password));

$answer = mysql_query("SELECT * FROM table WHERE username='".$user."'");
if(!mysql_num_rows($answer))
    die("User not found!<br />");

$donnees = mysql_fetch_array($answer);

if($password == $donnees['password'])
    // CONNECTION

?>