How to build an app that syns to website

I have a project in early stages but one feature will be a page that holds competition results from competitions that will be held.
I can create this easy enough in PHP and MySQL but what I really wanted was something that works something like Evernote (http://evernote.com/)
The idea is that the admin can either enter details on the website directly or they can download an app to iPad, iPhone, Android device etc and they can then enter the results of the competitions on the app. Then as soon as the device has access to the internet the competition results will automatically be synced to website and the online database

Any advice on where to start would be great
Thanks