Google Storage Incorrect Authorization Header with

I'm in the process of migrating from Amazon S3 to Google Storage and I can't seem to get my credentials to work. Here's some sample code that I put together to test my credentials:

$client = new S3Client([
    'credentials' => [
        'key' => 'GOOGxxxxxxxxxxxxxxx',
        'secret' => 'ZfcOTxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx',
    ],
    'region' => 'US',
    'version' => 'latest',
    'endpoint' => 'https://storage.googleapis.com',
]);

try {
    $result = $client->putObject(array(
        'Bucket' => 'devtest',
        'Key' => 'test',
        'Body' => 'Hello world'
    ));

    echo $result['ObjectURL'];
} catch (\Aws\S3\Exception\S3Exception $e) {
    // The AWS error code (e.g., )
    echo $e->getAwsErrorCode() . "\n";
    // The bucket couldn't be created
    echo $e->getMessage() . "\n";
}

Here's what I get back:

InvalidSecurity Error executing "PutObject" on "https://storage.googleapis.com/devtest/test"; AWS HTTP error: Client error response [url] https://storage.googleapis.com/devtest/test [status code] 403 [reason phrase] Forbidden InvalidSecurity (client): The provided security credentials are not valid. - InvalidSecurityThe provided security credentials are not valid. Incorrect Authorization header

I've tried googling 100 different combinations of this issue and can't find anything. I have Interoperability enabled, at least I think I do since I don't think I can get the key/secret without it being enabled first. And I have the Google Storage API enabled.

Any help would be greatly appreciated.

Edit: here's the Authentication Header in case that helps:

AWS4-HMAC-SHA256 Credential=GOOGGUxxxxxxxxxxx/20150611/US/s3/aws4_request, SignedHeaders=host;x-amz-content-sha256;x-amz-date, Signature=9c7de4xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

I noticed it stays "aws4_request" even when I specify 'signature' => 'v2'. Not sure if that matters.

I took a look at the S3Client code and it doesn't use the 'signature' config key as far as I can tell. The only thing I found was 'signature_version' which when set to v2, I get this error:

Unable to resolve a signature for v2/s3/US. Valid signature versions include v4 and anonymous.

I'm using Laravel 5.1 with composer package aws/aws-sdk-php version 3.0.3

Any ideas?

S3 only supports v4 signatures, and this requirement is enforced by the PHP SDK. It seems that Google Cloud Storage only supports v2 signing, so you wouldn't be able to use the same library to talk to both. Google does provide their own PHP SDK, which might make talking to Cloud Storage a bit easier.