Why does your browser try to save rather than open your PHP scripts?
PHP is a server-side scripting language, which means that scripts are interpreted and executed by the server before PHP pages are sent to the client (the browser). The server replaces PHP scripts with the output it obtains from their execution and then transmits to the client the output thus obtained. Hence, your browser cannot execute PHP scripts, but it can only request PHP pages from a server which runs the scripts.
Because the server is what processes the script, PHP needs to be installed on the server for the server to be able to process PHP scripts. If PHP isn’t installed, the server has no idea that “foo.php” is a PHP script so just sends it to the browser as an executable - in a way that forces the browser to prompt to download it.
Consult your hosting provider’s help for installing, setting up or enabling PHP. Alternatively, you can install it yourself if your host provides the permissions to (See the related questions for installation help). If your host does not provide PHP support, find another host.