How to create new button/functionality in Gmail via extension?

Question

How do you add a button/whatever in a compose message window in GMail so that on the click, it brings you to a popup form that queries data from a external MySQL DB, user selects criteria, hits send, and PHP on our side slices/dices the data and inserts it into our DB.

As a bit of a visual aid, I'd like to build a extension like https://www.youtube.com/watch?v=jm2WEa33RCg#t=10 where there's a button, you click it, it's getting the information from somewhere, but I have no idea how or where it's doing that.

Alternatively, is there a way one can create dropdown boxes or a insert a form when the user composes a message and get that data out?

The key functionality is the user needs to be able to access this drop down box of criteria which gets queried from the database from within Gmail. Otherwise, it's pointless.

I've been trying to figure out how to get the first part to work, and it seems like the only real method would be to develop extensions for all the major browsers.

Answer 1

You cant unless you write extensions and maintain them forever after. There is only an api for gmail contextual gadgets but those go in the received emails not on compose.