Here's a C library/API for Yahoo Messenger. URL: http://libyahoo2.sourceforge.net/

I want to use this API in my PHP/ASP script. But i dont know how to
use this. I know both C & PHP. But how to use this C code API in my
PHP code.

I also came across two Java APIs for the same purpose (jYMSG, Hamsa).
But i need the code to be in either PHP or ASP. 

I need step by step info on how to use this API.

I guess you'd have to build it into one of these...

http://www.zend.com/php/internals/extension-writing1.php

Or, create a seperate server component, and use php to access it like
a webservice or something..