実行をブロックせずに PHP で非同期 HTTP リクエストを作成するにはどうすればよいですか?

function post_without_wait($url, $params) {
    // Convert parameters to a string
    $post_string = http_build_query($params);

    // Parse the URL
    $parts = parse_url($url);

    // Open a socket connection
    $fp = fsockopen($parts['host'],
        isset($parts['port']) ? $parts['port'] : 80,
        $errno, $errstr, 30);

    // Construct and send the HTTP request
    $request = "POST " . $parts['path'] . " HTTP/1.1\r\n";
    $request .= "Host: " . $parts['host'] . "\r\n";
    $request .= "Content-Type: application/x-www-form-urlencoded\r\n";
    $request .= "Content-Length: " . strlen($post_string) . "\r\n";
    $request .= "Connection: Close\r\n\r\n";
    if (isset($post_string)) {
        $request .= $post_string;
    }

    fwrite($fp, $request);

    // Close the socket connection
    fclose($fp);
}