A number to keep track of what app is making calls to the API.
Should be kept secret. Not a big disaster if revealed.
With the Flickr and YouTube APIs above, you can build a browser app.
Does not have to be a standalone app.
Can be a web page with HTML and JS that makes the API call (e.g. using JSONP).
e.g. Can have a web page that in response to some event or user input can search YouTube for videos and return a list of videos
to put inside the page
without going to a new URL.
Browser apps and API secrecy
Problem: The web page
API key not secret.
Set up a simple proxy on your server.
This proxy calls the service with the API key (not exposed) and outputs the result.
Should be just a few lines of PHP.
Sometimes I link to Wikipedia.
I have written something
In defence of Wikipedia.
It is often a useful starting point
but you cannot trust it.
Linking to it is like linking to a Google search.
A starting point, not a destination.
highlight in red all
links to Wikipedia and Google search
and other possibly-unreliable user-generated content.