PHP class for the Google URL shortener API

Google-shortener.php

<?php  
if (!function_exists('curl_init')) 
    trigger_error('CURL is not installed');     
/* 
 * This program is free software: you can redistribute it and/or modify 
 * it under the terms of the GNU General Public License as published by 
 * the Free Software Foundation, either version 3 of the License, or 
 * (at your option) any later version. 
 *  
 * This program is distributed in the hope that it will be useful, 
 * but WITHOUT ANY WARRANTY; without even the implied warranty of 
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the 
 * GNU General Public License for more details. 
 *  
 * You should have received a copy of the GNU General Public License 
 * along with this program.  If not, see <http://www.gnu.org/licenses/>. 
 */ 

/** 
 * GoogleURL 
 *  
 * A simple class to replace all existing URL's with the Goo.gl URL's 
 *  
 * @author Bas van Dorst <info@basvandorst.nl> 
 * @version 1.0  
 * @package Google 
 */ 
class GoogleURL { 
     
    /** 
     * URL of the Google URL shortener API 
     * @var string 
     */ 
    private static $_api = "http://goo.gl/api/shorten"; 
     
    /** 
     * Curl timeout 
     * @var int 
     */ 
    private static $_curl_timeout = 5; 
     
    /** 
     * URL-regex (http://flanders.co.nz/2009/11/08/a-good-url-regular-expression-repost/) 
     * @var string 
     */ 
    private static $_urlregex = '/(?:(?:ht|f)tp(?:s?)\:\/\/|~\/|\/)?(?:\w+:\w+@)?(?:(?:[-\w]+\.)+(?:com|org|net|gov|mil|biz|info|mobi|name|aero|jobs|museum|travel|[a-z]{2}))(?::[\d]{1,5})?(?:(?:(?:\/(?:[-\w~!$+|.,=]|%[a-f\d]{2})+)+|\/)+|\?|#)?(?:(?:\?(?:[-\w~!$+|.,*:]|%[a-f\d{2}])+=(?:[-\w~!$+|.,*:=]|%[a-f\d]{2})*)(?:&(?:[-\w~!$+|.,*:]|%[a-f\d{2}])+=(?:[-\w~!$+|.,*:=]|%[a-f\d]{2})*)*)*(?:#(?:[-\w~!$+|.,*:=]|%[a-f\d]{2})*)?/'; 
     
    /** 
     * Replace existing URL with the Goo.gl URL. 
     *  
     * @param string|array $input_url 
     * @return string If the request succeeded, return Goo.gl URL else return the input URL. 
     */ 
    public static function shortURL($input_url)  
    { 
        $url = ( is_array( $input_url ) ) ? $input_url[0] : $input_url; 
         
        $post_fields = array(    "security_token" => "null", 
                                "url" => $url 
                            );   
          
        $ch = curl_init(); 
        curl_setopt($ch, CURLOPT_URL, self::$_api); 
        curl_setopt($ch, CURLOPT_HEADER, 0); 
        curl_setopt($ch, CURLOPT_POST, TRUE); 
        curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query( $post_fields ) ); 
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE); 
        curl_setopt($ch, CURLOPT_TIMEOUT, self::$_curl_timeout); 
        $output = json_decode( curl_exec($ch) ); // Google returns JSON-output 
        curl_close($ch); 
         
        return (isset( $output->short_url )) ? $output->short_url : $input_url; 
    } 
     
    /** 
     * Replace all normal links in a text/HTML-document to Goo.gl links 
     *  
     * @param string $input_text 
     * @return array|null 
     */ 
    public static function shortText($input_text)  
    { 
        return preg_replace_callback( 
                    self::$_urlregex,  
                    __CLASS__.'::shortURL',  
                    $input_text 
                ); 
    } 
}