There might be several ways to fix PHP file download problem, however, in most cases, this is caused due to .htaccess.
Here is how to fix it. Go to your Control Panel -> File Manager, get into your public_html folder and create new .htaccess file, add the following line to it:
AddType application/x-httpd-php .php .htm .html AddHandler x-httpd-php .php .htm .html
Now simply save the file and reload your website, it should work fine now!