Hi Siva -
You didn't really ask a question here. Just told us what you're doing. What is your question?
You won't want to pass the Tableau username and password to Tableau from your PHP page.. Instead, you want to use Trusted Tickets (also known as Trusted Authentication) to get a "ticket" from Tableau Server once YOUR PHP webapp has authenticated the user using whatever mechanism you choose. That ticket can be used to view content in Tableau later on.
Please read the following help topics and then let us know if you have any questions: