You can use a socket. See this example.
Edit:
Here is the code from the above link:
// Example:
// post_async("http://www.server.com/somewhere",array("foo" => 123, "bar" => "ABC"));
function post_async($url, $params)
{
// Build POST string
foreach ($params as $key => &$val) {
if (is_array($val)) $val = implode(',', $val);
$post_params[] = $key.'='.urlencode($val);
}
$post_string = implode('&', $post_params);
// Connect to server
$parts=parse_url($url);
$fp = fsockopen($parts['host'],isset($parts['port'])?$parts['port']:80,$errno, $errstr, 30);
// Build HTTP query
$out = "$type ".$parts['path']." HTTP/1.1
";
$out.= "Host: ".$parts['host']."
";
$out.= "Content-Type: application/x-www-form-urlencoded
";
$out.= "Content-Length: ".strlen($post_string)."
";
$out.= "Connection: Close
";
$out.= $post_string;
// Send data and close the connection
fwrite($fp, $out);
fclose($fp);
}