Google Cloud signed URL with PHP - php

Question

My needs are simple... One would think. I'm just trying to create a signed URL with a PHP script to prevent download abuse of a file hosted on Google Cloud Storage. I've found several examples, none that worked. This one came the closest:
<?php
$filepath = 'my file.zip'; // do not include the bucket name, no slash in the beginning
$bucket = 'mybucket';
$key = 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXX'; // key
$secret = 'xxxxxxxxxxxxxxxxxxxxxxx'; // secret
function getSignedGoogleCloudStorageUrl($filepath, $bucket, $secret, $key, $duration = 300)
{
$expires = new \DateTime('+ ' . $duration . ' seconds');
$seconds = $expires->format('U');
$objectPieces = explode('/', $filepath);
array_walk($objectPieces, function (&$piece) {
$piece = rawurlencode($piece);
});
$objectName = implode('/', $objectPieces);
$resource = sprintf(
'/%s/%s',
$bucket,
$objectName
);
$headers = []; // you may add any headers needed here
$toBeSignedArray = [
'GET',
'', // contentMd5, can be left blank
'', // contentType, can be left blank
$seconds,
implode("\n", $headers) . $resource,
];
$toBeSignedString = implode("\n", $toBeSignedArray);
$encodedSignature = urlencode(base64_encode(hash_hmac('sha1', $toBeSignedString, $secret, true)));
$query = [];
$query[] = 'GoogleAccessId=' . $key;
$query[] = 'Expires=' . $seconds;
$query[] = 'Signature=' . $encodedSignature;
return "https://storage.googleapis.com/{$bucket}/{$filepath}?" . implode('&', $query);
}
echo getSignedGoogleCloudStorageUrl($filepath, $bucket, $secret, $key);
?>
However, it always produces the same result after the 1st use until I clear the cache on the web server.
Any help or other working examples would be great. Thanks.