My Googl URL shortener PHP library

Question

I knocked this together in a few hours as I was just simply bored. I was going to add a namespace to this but decided there was no point.
This is also available on Github

All feedback is appreciated!

<?php
/**
 * This class will allow you to generate goo.gl links
 * @package     Google
 * @subpackage  Googl
 * @author      Lloyd Davies
 * @version     1.1.0
 */
class Googl
{
  /**
   * @const (string) API_KEY
   * This constant stores the Google API key
  */
  const API_KEY = "YOUR_GOOGLE_API_KEY";

  /**
   * @const (string) API_URL
   * This is the Google API URL to be used with predefined placeholder (%s)
  */
  const API_URL = "https://www.googleapis.com/urlshortener/v1/url?key=%s";

  /**
   * @const (boolean) DEBUG
   * A boolean value to indicate to display errors/exceptions
  */
  const DEBUG = true;

  /**
   * @const (string, array) ERROR_CODES
   * Source: https://developers.google.com/drive/v3/web/handle-errors
   * Stores a friendly defonition for error codes
  */
  const ERROR_CODES = [
    400 => 'Error: Missing required parameters, most likely key',
    401 => 'Error: Invalid API key',
    403 => 'Error: Daily limit exceeded',
    500 => 'Error: Internal Google API error'
  ];

  /**
   * @var (string) largeURL
   * This is the URL which is going to be converted
  */
  public $largeURL = null;

  /**
   * __construct
   * Magic method to inititate our class
   * Check that the environment is suitable
  */
  public function __construct($targetURL = null)
  {
    (!is_null($targetURL)) ? $this->largeURL = $targetURL : null;
    if (!function_exists("curl_init"))
    {
      throw new Exception("Error -> the PHP cURL library is not installed", 1);
    } elseif (empty(self::API_KEY) || strlen(self::API_KEY) !== 39)
    {
      throw new Exception("Error -> invalid Google API key", 1);
    }
  }

  /**
   * Generate
   * This will return the shortend URL
   * @return (string) - the shortend long URL
  */
  public function Generate()
  {
    $getResponse = $this->postRequest(
      $this->generateURL(),
      $this->createJSON($this->largeURL)
    );
    $json = json_decode($getResponse, true);
    if($this->errorExists($json))
    {
      $errorCode = (int)$json['error']['errors']['code'];
      if(self::DEBUG)
      {
        throw new Exception($this->getFriendlyErrorCode($errorCode), 1);
      }
    } else {
      return $json['id'];
    }
  }

  /**
   * createJSON
   * Will create the JSON payload needed to send to Google
   * @return (string) the serialized JSON string
   * @param (string) $longURL - the URL to be converted
  */
  private function createJSON($longURL)
  {
    return json_encode([
      'longUrl' => $longURL
    ]);
  }

  /**
   * postRequest
   * This will create a new cURL instance and inititate a POST request
   * @return (string) the response from the request
   * @param (string) $url - the URL to be requested
   * @param (string, array) $data - the POST data
  */
   private function postRequest($url, $data)
   {
     try
     {
       $ch = curl_init();
       $options = [
         CURLOPT_URL => $url,
         CURLOPT_HEADER => false,
         CURLOPT_SSL_VERIFYPEER => false,
         CURLOPT_FOLLOWLOCATION => 1,
         CURLOPT_RETURNTRANSFER => true,
         CURLOPT_USERAGENT => "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)",
         CURLOPT_CONNECTTIMEOUT => 10,
         CURLOPT_HEADER => 0,
         CURLOPT_TIMEOUT => 10,
         CURLOPT_POST => 1,
         CURLOPT_POSTFIELDS => $data,
         CURLOPT_HTTPHEADER => ['Content-Type: application/json']
       ];
       curl_setopt_array($ch, $options);
       $response = curl_exec($ch);
       curl_close($ch);
       return $response;
     } catch (Exception $e) {
       if(self::DEBUG)
       {
         throw new Exception(
            $e->getMessage(),
            $e->getCode()
         );
       }
     }
   }

   /**
    * generateURL
    * This will generate the API URL to be used in the POST request
    * @return (string) - the generated URL
   */
   private function generateURL()
   {
     return sprintf(
        self::API_URL,
        self::API_KEY
     );
   }

   /**
    * getFriendlyErrorCode
    * @param (int) $errorCode - the error code returned
    * @return (string) - the friendly error message
   */
   private function getFriendlyErrorCode($errorCode)
   {
     return (array_key_exists(self::ERROR_CODES, $errorCode)) ? self::ERROR_CODES[$errorCode] : "Undefined error";
   }

   /**
    * errorExists
    * @param (string, array) $jsonArray - the JSON object
    * @return (bool) - if it exists or not
   */
   private function errorExists($jsonArray)
   {
     return array_key_exists('error', $jsonArray);
   }
}