Browser Tries to Download the install.php file

I just set up a new server running Linux operating system and Plesk 10.3.1 control panel.

I uploaded all WordPress files to my server. But when I attempted to install WordPress using domainname/wp-admin/install.php, the browser tried to download the install.php file instead of displaying the WordPress installation page.

Any ideas how I can fix the problem?

I am a newbie, so please be non-technical.

Thank you very much for the help.