I have a site where customers can place online orders. I want that whenever a order is placed a invoice is also added in Quickbook for the placed order.
I have installed QuickBooks PHP DevKit (https://github.com/consolibyte/quickbooks-php) at http://res-pos.com/quickbooks-php/docs/partner_platform/example_app_ipp_v3/ but the problem with it is that i need be connected with Quickbooks using the "Connect to QuickBooks" button manually.
In my scenario i need it automated so that there would be no need to connect manually.
You are totally misunderstanding how OAuth works and haven't read Intuit's documentation (or, for that matter, any other OAuth documentation).
You need to connect to QuickBooks manually ONCE and only ONCE!
After you've connected once you never ever ever need to connect manually ever again!
Connect once, and be done with it, and then it's all automated from there on out.