How to use open id as login system

Thanks to some other comment on Stackoverflow.com I came to learn about LightOpenId. It is really easy to use.

The example code just works(without any configuration):

<?php
require 'openid.php';
try {
    $openid = new LightOpenID;
    if(!$openid->mode) {
        if(isset($_POST['openid_identifier'])) {
            $openid->identity = $_POST['openid_identifier'];
            header('Location: ' . $openid->authUrl());
        }
?>
<form action="" method="post">
    OpenID: <input type="text" name="openid_identifier" /> <button>Submit</button>
</form>
<?php
    } elseif($openid->mode == 'cancel') {
        echo 'User has canceled authentication!';
    } else {
        echo 'User ' . ($openid->validate() ? $openid->identity . ' has ' : 'has not ') . 'logged in.';
    }
} catch(ErrorException $e) {
    echo $e->getMessage();
}

Using google as openid provider.

<?php
# Logging in with Google accounts requires setting special identity, so this example shows how to do it.
require 'openid.php';
try {
    $openid = new LightOpenID;
    if(!$openid->mode) {
        if(isset($_GET['login'])) {
            $openid->identity = 'https://www.google.com/accounts/o8/id';
            header('Location: ' . $openid->authUrl());
        }
?>
<form action="?login" method="post">
    <button>Login with Google</button>
</form>
<?php
    } elseif($openid->mode == 'cancel') {
        echo 'User has canceled authentication!';
    } else {
        echo 'User ' . ($openid->validate() ? $openid->identity . ' has ' : 'has not ') . 'logged in.';
    }
} catch(ErrorException $e) {
    echo $e->getMessage();
}