AMP update cache, followup the google tutorial: https://developers.google.com/amp/cache/update-cache#call-the-update-cache

amp-update-cache.php

<?php
require __DIR__ . '/vendor/autoload.php';
# https://github.com/phpseclib/phpseclib
use phpseclib\Crypt\RSA;

$update_cache_url = $_GET['url'] ?? $_GET['link'] ?? '';

if( empty ($update_cache_url) || strpos($update_cache_url, 'https://www.sample.com.au/') === false ) {
	die("Please provide a valid url, given: " . $update_cache_url);
}

$private_key_path = __DIR__ . 'private-key.pem';

$rsa = new RSA();
$rsa->loadKey($private_key_path);

$plaintext = '/update-cache/c/s/' . $update_cache_url . '?amp_action=flush&amp_ts=' . time() ;

$rsa->setSignatureMode(RSA::SIGNATURE_PKCS1);
$signature = $rsa->sign($plaintext);
$base64_signature = base64url_encode( $signature );

$amp_request_update_url = 'https://www-sample-com-au.cdn.ampproject.org/c/s/' . $update_cache_url . $plaintext . '&amp_url_signature='.$base64_signature;

$response = get_response( $amp_request_update_url);

echo json_encode( $response );
exit(0);

function get_response($url) {
	// init curl object
	$curl_object = curl_init();

	// define options
	$options = array(
		CURLOPT_URL => $url,
		CURLOPT_RETURNTRANSFER => true,
	);

	// apply those options
	curl_setopt_array($curl_object, $options);

	// execute request and get response
	$result = curl_exec($curl_object);

	return $result;
}

function base64url_encode($data) {
	return rtrim(strtr(base64_encode($data), '+/', '-_'), '=');
}