A payment gateway involves the use of technology / security whose scope is not within the confines of this post. Did you mean to ask how to integrate a website / web page with an existing payment gateway ? The gateways' documentation is the way to go.
please help me with an example.
I doubt one will be provided by anyone given that your post contains very few details. ShowSomeEffort
This is a 4 year old thread and it is very unlikely that the people in the thread will respond now.
Deepak has already posted a link which explains almost everything realted to PayPal integration.
Also it can be integrated with Drupal.
I am locking this thread. If you have further questions, please start a new topic in the appropriate forum.