A facebook tab is a page on your (own) server framed in by facebook. So create a html page, make it available under http and https. Next follow the instruction in this tutorial:
https://developers.facebook.com/docs/appsonfacebook/pagetabs/
create your page
create a new app developers.facebook.com
choose page tab
- install your app by visting:
https://www.facebook.com/dialog/pagetab?app_id=YOUR_APP_ID&next=YOUR_URL
You don't need to use the facebook php-sdk or api's(graph api) to do this!