Need help with disaster management app

We want to make a disaster (Earthquake) management and response application. We understand that similar applications (Sahara Eden, Ushahidi, etc.) already exist BUT here is what we want to be different than those existing apps:

(1) We want to use an interactive map in which users can view/mark/edit/update critical sites such as rescue centeres, blocked roads, collapsed buildings, etc. by just cliking on the map. For example, 1st click gives them a menu to select what sort of critical site to mark and the 2nd click (on the menu-item) actually places it on the map (where the 1st click was made.)
(2) The changes made by one user need to be persistent so that other users can see it too. So we need a back-end database where coordinates of critical sites need to be stored into and retrieved from.
(3) This app will have two front ends - Web and Android.
(4) Both front-end apps will use a basemap from the map provider (Google or OpenStreetMap, etc.) and will connect to the back-end database (most preferably MySQL)

So, our questions are:
(1) Given the requirements above, what will be the best way to go about it?
(2) Which basemap provider and map API (cloudmade, Leaflet, etc.) should we use to achieve above map interactions?
(3) We know PHP and MySQL so we'd like to stick to these, if possible. Your take on that?