Why is it not showing any output?

See more:

PHP

API

cURL

web_application

Can someone help me to understand how to do this API thing? The problem is, it doesn't display an output. Then, I have learned that the GET is not really needed(can be there or not) to be part of the code, but in this case, when I am deleting the GET I am getting an error output. This:

<a href="https://support.dudamobile.com/API/REST-Resources/Analytics">link</a>

is what I need to do...(please see the link)

So far, I have this code...

<?php
$data = '';
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'GET https://api.dudamobile.com/api/analytics/site/{site_name}?from=2015-01-12&result=activities');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_USERPWD, "{username}:{password}");
curl_setopt($ch, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(                                                                          
    'Content-Type: application/json',       
    'Content-Length: ' . strlen($data))                                                                       
);   
$output = curl_exec($ch);
curl_close($ch);
print_r($output);
?>