Amp cache url PHP

amp-update-cache-url-generator.php

<?php

function urlsafe_b64encode($string) {
    return str_replace(array('+','/','='),array('-','_',''), base64_encode($string));
}

$timestamp=time();
$ampBaseUrl = "https://www-domain-com.cdn.ampproject.org";
$signatureUrl = '/update-cache/c/s/www.domain.com/amp/page-url?amp_action=flush&amp_ts='.$timestamp;

// opening the private key
$pkeyid = openssl_pkey_get_private("file://amp-private-key.pem");

// generating the signature
openssl_sign($signatureUrl, $signature, $pkeyid, OPENSSL_ALGO_SHA256);

openssl_free_key($pkeyid);
// urlsafe base64 encoding
$signature = urlsafe_b64encode($signature);

// final url for updating
$ampUrl = $ampBaseUrl.$signatureUrl."&amp_url_signature=".$signature;

echo $ampUrl."\n";

@krzysztofbukowski Dont work for me... i get always "URL signature verification error"

@krzysztofbukowski Add this line: $ampUrl = str_replace("&", "&amp", $ampUrl); before echo $ampUrl."\n";

work now!

Dont work for me...
openssl_sign(): supplied key param cannot be coerced into a private key in

@krzysztofbukowski The value of $signature is empty for me.

I generated a private key, and placed in the same directory as the PHP script. In fact, I coded
$pkeyid = openssl_pkey_get_private("/full/path/to/private-key.pem");

If I echo the value of $signature, it's empty.