Curl is the way to go for simple requests. It runs cross platform, has a PHP extension and is widely adopted and tested.
I created a nice class that can GET and POST an array of data (INCLUDING FILES!) to a url by just calling CurlHandler::Get($url, $data) || CurlHandler::Post($url, $data). There's an optional HTTP User authentication option too :)
/**
 * CURLHandler handles simple HTTP GETs and POSTs via Curl 
 * 
 * @package Pork
 * @author SchizoDuckie
 * @copyright SchizoDuckie 2008
 * @version 1.0
 * @access public
 */
class CURLHandler
{
    /**
     * CURLHandler::Get()
     * 
     * Executes a standard GET request via Curl.
     * Static function, so that you can use: CurlHandler::Get('http://www.google.com');
     * 
     * @param string $url url to get
     * @return string HTML output
     */
    public static function Get($url)
    {
       return self::doRequest('GET', $url);
    }
    /**
     * CURLHandler::Post()
     * 
     * Executes a standard POST request via Curl.
     * Static function, so you can use CurlHandler::Post('http://www.google.com', array('q'=>'StackOverFlow'));
     * If you want to send a File via post (to e.g. PHP's $_FILES), prefix the value of an item with an @ ! 
     * @param string $url url to post data to
     * @param Array $vars Array with key=>value pairs to post.
     * @return string HTML output
     */
    public static function Post($url, $vars, $auth = false) 
    {
       return self::doRequest('POST', $url, $vars, $auth);
    }
    /**
     * CURLHandler::doRequest()
     * This is what actually does the request
     * <pre>
     * - Create Curl handle with curl_init
     * - Set options like CURLOPT_URL, CURLOPT_RETURNTRANSFER and CURLOPT_HEADER
     * - Set eventual optional options (like CURLOPT_POST and CURLOPT_POSTFIELDS)
     * - Call curl_exec on the interface
     * - Close the connection
     * - Return the result or throw an exception.
     * </pre>
     * @param mixed $method Request Method (Get/ Post)
     * @param mixed $url URI to get or post to
     * @param mixed $vars Array of variables (only mandatory in POST requests)
     * @return string HTML output
     */
    public static function doRequest($method, $url, $vars=array(), $auth = false)
    {
        $curlInterface = curl_init();
        curl_setopt_array ($curlInterface, array( 
            CURLOPT_URL => $url,
            CURLOPT_RETURNTRANSFER => 1,
            CURLOPT_FOLLOWLOCATION =>1,
            CURLOPT_HEADER => 0));
        if (strtoupper($method) == 'POST')
        {
            curl_setopt_array($curlInterface, array(
                CURLOPT_POST => 1,
                CURLOPT_POSTFIELDS => http_build_query($vars))
            );  
        }
        if($auth !== false)
        {
              curl_setopt($curlInterface, CURLOPT_USERPWD, $auth['username'] . ":" . $auth['password']);
        }
        $result = curl_exec ($curlInterface);
        curl_close ($curlInterface);
        if($result === NULL)
        {
            throw new Exception('Curl Request Error: '.curl_errno($curlInterface) . " - " . curl_error($curlInterface));
        }
        else
        {
            return($result);
        }
    }
}
?>
[edit] Read the clarification only now... You probably want to go with one of the tools mentioned above that automates stuff. You could also decide to use a clientside firefox extension like ChickenFoot for more flexibility. I'll leave the example class above here for future searches.