Tedber on "wp-admin/install.php"

I have installed wp on ubuntu 10.04 nginx as admin tool for bigbluebutton.
It seems as if the installation of bbb, mysql, php5 and wp3.5.1 went ok.
my problem is; by just wiriting on firefox browser <localhost/wp-admin/install.php> the php scrypt is not running instead i see all the texts of the install.php file.
I've access to bbb demo page.
I am working on the server. It is my local machine using private IP.
can any one help me to fix this failure?
thank you.
My knowledge level of linux is beginner.