How can I build an API in PHP

Hello I am looking to build a basic API and application/apps system on my social network something like Facebook or other sites have, my site is in php/mysql. Here are some questions.

1)

Basically what I want to do is give a user a API key and secret. After I have these how can I use them in my php app to authenticate a user request which would come from there server?

2) I can basically build an API to send a request to my server with CURL and get the result back to the page in XML or JSON or whatever, the problem is when sending the request the user would have to know the user ID they want to send to lookup data aga开发者_StackOverflow社区inst, this is fine for an API but I am wanting to have an Apps section where the user's app site would be using the API and would be loaded into my site in the app section with an iframe, the problem is, I need to find a good way to make it where a logged in user on my site can go to the app section and go to an app and there username should be available to that page loaded in the iframe. Facebook and myspace do this somehow and many other sites; I am lost how to get around this hurdle.

Any help on any of this is really appreciated, thank you

Update:

I just had an idea, if I require a cookie to be set when a user visit's my site, then they would have a cookie and it could hold there User ID, then my API script could look for that cookie to exist and grab it's value?

If you plane on using an IFRAME, then no, your API hosted on a separate website (the website inside the IFRAME) would not be able to grab the cookie. The cookie is only visible on the website that it was set for.

I have only used Facebook API with the FBML (not the IFRAME,) but all they do their is basically replace what's in the page with the info that the "tag" is calling. I'm not sure if there is a better way, but you could possibly call a page on the app's server (say the app is hosted at http://example.com/app/, and you called http://example.com/app/?id=28318&name=John%20Maguire,) and have your API code handle it and turn it into a variable?

Maybe you should look into the source code of the Facebook API client.