Google Cloud Storage Signed URLs — The Easy Way

<?php

$filepath = 'path/to/file.tar.gz'; // do not include the bucket name, no slash in the beginning
$bucket = 'my-bucket-name';
$key = 'GOOGSOMEKEYHERE'; // key
$secret = 'aNdTHEsecretGoesRitghtHere'; // secret

function getSignedGoogleCloudStorageUrl($filepath, $bucket, $secret, $key, $duration = 50)
{
    $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);
}