default to index.html but allow user to type to index.php and get the php page

I’m busy building a WordPress website for a client. I’ve developed the site off-line and would now like to upload it to the live server to show the client what I’ve done so far. I want to keep the “Under Construction” html page I’ve create as the default when browsing to the site but if you type in https://www.sitename.com/index.php I want the actual site to be displayed.

I don’t want to use an “under construction” plugin as I don’t want the user to have to enter any form of username or password to gain access to the site.

I have tried: DirectoryIndex index.htm but as soon as I browse to index.php it flips me back to the index.htm page.

My development machine runs Windows 8 with XAMPP.

Please give detailed code suggestions, I’m really not familiar with .htaccess so you need to be very clear.