In http://stackoverflow.com/questions/962915/how-do-i-make-an-asynchronous-get-request-in-php, it shows how to send asynchronous request from PHP script, so that the webpage is immediately rendered without waiting the request to finish.

This technique is important if you want to implement background processing in web app, just by send a request to own script that going to run the process at background, while the main script continue to run and produce the webpage.

However, the request made to the background process page maybe can be accessed directly, just by entering the correct URL to browser, and you want to make sure this page is not being abused by users.

During sending the request, we can modify the request header as an authentication method to check if request is originally come from our own web application. You may modify user agent header and set it as your own webapp UA

‘User-Agent: my-web-app’, and in your script, check the value of $_SERVER[‘HTTP_USER_AGENT’]

Or, for more security, use custom header name, as follows:

$host = 'localhost';
$path = '/bg.php';
$qs = array(); // query string
if (!empty($qs)) {
    $qs = http_build_query($qs);
    $path = $path .'?'. $qs;
}
$fp = fsockopen($host, 80, $errno, $errdesc);
if ($fp) {
    $req  = "GET $path HTTP/1.0\r\n";
    $req .= "Host: $host\r\n";
    $req .= "Content-Type: application/x-www-form-urlencoded\r\n";
    $req .= "Content-Length: ". strlen($qs) ."\r\n";
    $req .= "Anxx0Wjoiw3: asmkd3A0das2wq2\r\n";
    $req .= "Connection: Closern\r\n";
    fputs($fp, $req);
    fclose($fp);
}

And in the background process script, check if the custom header is correct:

if (isset($_SERVER['HTTP_ANXX0WJOIW3']) && $_SERVER['HTTP_ANXX0WJOIW3'] == 'asmkd3A0das2wq2') {
    // do background process
}

Note: Custom header (or other headers), can be access using $_SERVER variables, with the key prepended with ‘HTTP_’

As an addition, use encrypted value for the header name and value, and also change the value for every several hour by autogenerate it using mktime() and strtotime(), provided you secure the encrypted string with salt data, example:

$salt = 'secret-key';
$hash = md5($salt . mktime(date('H', strtotime('+6 hour')), 0, 0));