PHP goo.gl url shortener

Here's an implementation of Google's url shortener: goo.gl.

<?
class googl {
 const api = 'https://www.googleapis.com/urlshortener/v1/url';
 private $key = null;
 
 public function __construct($key = null) {
  if(defined('GOOGLE_API_KEY')) {
   $this->setKey(GOOGLE_API_KEY);
  }
  
  if(!is_null($key)) {
   $this->setKey($key);
  }
 }
 
 public function setKey($key) {
  $this->key = $key;
 }
 
 public function s($url) {
  $data = $this->shorten($url);
  return isset($data->id) ? $data->id : $url;
 }
 
 public function shorten($url) {
  $key = '';
  $data = array();
  $data['longUrl'] = $url;
  
  if(!is_null($this->key)) {
   $key = '?key='.$this->key;
  }
  
  return $this->fetch(self::api.$key,$data);
 }
 
 public function expand($url) {
  $key = is_null($this->key) ? '' : "&key={$this->key}";
  return $this->fetch(self::api.'?shortUrl='.urlencode($url)."$key&projection=FULL");
 }
 
 private function fetch($url, $data = array()) {
  $ch = curl_init();
  
  curl_setopt($ch, CURLOPT_URL, $url);
  curl_setopt($ch, CURLOPT_HEADER, 0);
  curl_setopt($ch, CURLOPT_AUTOREFERER, 1);
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
  curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 10);
  curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-type: application/json'));
  
  if(!empty($data)) {
   curl_setopt($ch, CURLOPT_POST, 1);
   curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($data));
  }
  
  $r = curl_exec($ch);
  curl_close($ch);
  
  return json_decode($r);
 }
}
?>