Install Not working – files downloading not opening

I have installed WP on my domain using a one click installer, using Softaculous. It has created the database and appears to have created all the settings correctly in the files. I cannot open any of the files on the site – all of my browsers – Safari, Chrome, Firefox (MacOSX) Download PHP files rather than open or run anything on the site.

Any ideas? Have contacted the ISP who say it is working fine but I cannot get anything other than the readme.html page to open.

Grateful for any advice