Ok i will from what i see you are not familiar with APIs. You access the information using cURL
there is a good page in the documentation that gives you examples.
Now for using curl it's fairly simple in php:
$url = 'https://external.ningapis.com/xn/rest/apiexample/1.0/Photo/recent?xn_pretty=true'
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_HEADER, FALSE);
curl_setopt($ch, CURLOPT_NOBODY, TRUE); // remove body
curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
$response = curl_exec($ch);
$httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
curl_close($ch);
Now you have all the information you need inside $response
Check the API documentation for the exact way to work with this API. also check curl on php.net