How to interface with web XML/JSON APIs?

In PHP you might have something like this:

// EDIT: only need to use urlencode() on user supplied variables//$url = urlencode("http://xyz.com/api?apikey=foo&v1=bar&v2=baz");$url = "http://xyz.com/api?apikey=foo&v1=bar&v2=baz";$response = file_get_contents($url);

The $response will contain a string of whatever xyz.com outputted when you accessed $url (it's what you would see if you visited $url directly).

Your next job would be to parse $response based on its data structure (e.g XML, JSON, etc) so that it's usable by the rest of your code.

There are several PHP libraries for parsing XML or JSON. Personally, I prefer to use SimpleXMLElement and json_decode() which is included with PHP 5 >= 5.2.0.

Depending on the API, it will probably send you some sort of error code/response structure if it doesn't understand the request $url which you could check for after you parse the response.

If $response returns false, then typically there was some error communicating with the $url.

I found that an intuitive way to think about these XHR requests is that you're passing arguments (GET parameters) to a function (API URL). And the response from the API URL is like the return statement from a function.

UPDATE:

API example for Groupon as suggested by OP in comments:

$apikey = "client_id=abcd1234567890";$division = "division_id=chicago";$url = "http://api.groupon.com/v2/deals?" . implode("&", array($apikey, $division));$response = file_get_contents($url);$deals = json_decode($response, true);foreach($deals['deals'] as $deal){    $format = 'Deal: <a href="%s">%s</a><br/>';    echo sprintf( $format, $deal['dealURL'], $deal['announcementTitle']);}

The above code would print out a listing of all deal titles and urls for the Chicago area. If you look at the Sample JSON Response sections on the Groupon API page, it will give you the entire data structure that would be mapped to the associative array $deals.

If any of the GET parameters to the API are provided by the user (e.g. from a web form), you will want to do something like $division = "division_id=" . urlencode($user_input);.