Please follow the following steps to Invite friends from yahoo mail.

Fist of all you need application Id, consumer key and consumer secret form yahoo. To generate API key and Consumer secret please login to Yahoo Developer Network. open http://developer.yahoo.com/oauth/ and click on Create a Project, fill up the form and you can generate the required information.

Files Included in this project.

config.php

In this file you have to enter the generated  keys and the success URL

<?php
$app_id = 'YOUR APP ID';
$consumer_key = 'YOUR CONSUMER KEY';
$consumer_secret= 'YOUR CONSUMER SECRET';
$connected_path = 'connected.php';
$re_url = "YOUR DOMAIN NAME/connected.php";
?>

connected.php

Once the process completed this file will show you the list of friend from your yahoo address book. From here you can send message to all your friends or selected friends.

Also you need some external file like Yahoo.inc and YahooSessionStore.inc, You will get all required files from attached zip file.

Run connected.php file, it will ask you to login in to Yahoo mail, if you are already login then it will as you for permission.

http://www.yourdomain.com/connected.php

Once you gave permission to access your content, it will redirect to connected.php file, where it will show you the list of friends and now you can save the list to your database or send mail etc.