<?php
// Connect to the DB
$dbuser = 'sqluser';
$dbpass = 'sqlpass';
$db = mysql_connect("localhost","$dbuser","$dbpass");
mysql_select_db("users",$db);
$query = mysql_query("SELECT * FROM users WHERE username = '$_POST[username]'");

while($array = mysql_fetch_assoc($query)) {
    if (md5($_POST[password]) != $array[password]) {
        die("Incorrect password.");
    } else {
        $_SESSION['username'] = "$_POST[username]";
    }
}
?>