There will be many zoom levels all generated by PHP when the admin clicks export. You can make it the way you feel is best, but I think different levels of "tiles" would be the best way (stored as images) and peaced together to be viewed.
map view side:
The admin can define the display size of a map (like 400px x 600px) and the starting point of the map (where it will center over). The admin can also set a boundaries that the user can view outside of (top left xy, bottom right xy). The user can zoom in and zoom out and scroll north, north-east, east, south-east, south, south-west, west, north-west. The user can also click on the map to zoom in. Once the user is at a point where an acre can be determined (last few zoom levels) when the user clicks on a acre an event will fire (in JS or however you see fit) that will send a request to the server telling it where the user clicked. This will allow for building placement, troop movement, and so on. The server will respond by updating the map (using PHP to draw on the selected acre) and the user will see the change real-time. if the user is happy and clicks a button, like "move troops" (the button part will be done by me) some event must be able to trigger the saving of that map drawing so that everyone who is viewing the map would see the change.
The map should be updating real-time. if a Acre changes while I am viewing the map, the new images should be displayed to the user.
There is also the possiblity of loading a map, then placing (drawing) acre (tile) changes specific to that viewer (user). For example if user A is moving his troops to xy from xy... only user A should be able to see that... only changes to the map that "everyone" can see should be stored to an image (if we have 10,000 people playing, it would take a lot of resources to generate that image every time they view it.)
If you have a better design / changes to this plan, you are welcome to suggest them. I will not force any "method" of completing the task at hand...
Must be able to support IE and Firefox / with and without Ajax.
Needs to be as custmizable as possible.
If needed, I will give you access to a FTP account with a real URL and database for testing / working.
The basic concept is that the admin will start creating maps... in the end the map will be the entire world. User will be clicking on the map to order large troop movments. (from point A to pont B) only the user can see there troops moving on the map. The user will be able to zoom in and out to view the world. There will be different "kingdoms" located around the world (that is why I need to be able to draw on the map as it's updated, for example, a kingdom expands it's boards, this needs to be displayed on the map real-time).
This should be able to support 10,000+ users.