I have seen many Places even i searched a lot before using it that is there any example for PHP to retrieve the data using API here are few examples i am posting but few things needed:

1. Make Sure you have the API Key which you can get by going in Settings>Channels
   
2. Apply for the API Client in the API section clicking "Add API client" button fill the form and it will generate a API Key and some other useful information
3. Now Open your PHP File Lets say you want to get all the articles on your Website using API then you can use following code:

$r = new HttpRequest('https://YOUR_DOMAIN_NAME/api/v1/articles.json', HttpRequest::METHGET);
$r->addQueryData(array('client' => 'INSERTYOURAPIKEY_HERE'));
try {
$r->send();
if ($r->getResponseCode() == 200) {

                           $obj=json_decode($r->getResponseBody());
                           print_r($obj);

                   }
        } 
catch (HttpException $ex) {
           echo $ex;
         }

Make Sure you Put Relevant Info Like your Domain name and API KEY to run this code