I want to write a PHP code (or other code like perl ...) that send a yahoo PM to my friend . may you help me in algorithm , about how to connect to yahoo server , how to send the pm ... Thanks .
Yahoo's developer site should be a good start. For example, a PHP library for Yahoo Mail is linked there.
With the Mail API, you can build applications that display message summary information, parse message contents, manage folders, and even compose and send messages.
Client libraries in Python, PHP and VB.NET are available to make using the Mail developer service easier.
This could be a good start to do something similar with the Yahoo Messenger API.
More searching reveals the Yahoo Messenger PHP SDK and libpurple, a library for IM access.