Amazon S3 PHP SDK pre-signed requests

I am trying to generate a presigned request with the S3 PHP SDK like this:

$s3Client = Aws\S3\S3Client::factory([
    'credentials' => new Aws\Common\Credentials\Credentials('my-access-code', 'xxx')
]);

$command = $s3Client->getCommand('GetObject', [
    'Bucket' => 'my-bucket-name',
    'Key' => 'awesome-cat-image.png',
]);

$signedUrl = $command->createPresignedUrl('+10 minutes');

But when I goto the URL I get an error saying:

<Error>
    <Code>SignatureDoesNotMatch</Code>
    <Message>
        The request signature we calculated does not match the signature you provided. Check your key and signing method.
    </Message>
    <AWSAccessKeyId>xxx</AWSAccessKeyId>
    <StringToSign>GET 1422564095 /my-bucket-name/awesome-cat-image.png</StringToSign>
    <SignatureProvided>xxx</SignatureProvided>
    <StringToSignBytes>
        xxx
    </StringToSignBytes>
    <RequestId>xxx</RequestId>
    <HostId>
        xxx
    </HostId>
</Error>

Accessing http://ift.tt/1JPs13g works just fine with permissions set to allow non authenticated users.