i want to change (.php) to my own extension like (.abc) and it should work like php page in lighttpd server in ubuntu os.
Anybody knows how to do this?

Why not? Example:

<?php
 echo 'hello world!';
?>

save it as hello.abc then run it like this.
php hello.abc

In terminal it will work.I know that.But i am talking about browser view about a website page.

It also works there. Your web server calls the scripts so you configure your site to use the new extension. This is not a PHP issue at all.

I donot know how to configure that in lighttpd.I have tried different methods but it dnt work for me.I have tried using adding some codes in (.htaccess) file.but it didnot work for me.

Time to hit the documentation?

Do you see where to add your .abc to where in the docs at https://redmine.lighttpd.net/projects/1/wiki/TutorialConfiguration

I can't tell you this as I want to know if you understand the basics. You must figure this out since you're going to have to run this non-standard setup.

Frankly, don't do this. No good reason given so far but as an exercise, you get to dive into the docs of lighttpd and next maybe some PHP config.