How to unlock the file after AWS S3 Helper uploading file?

I am using the official PHP SDK with the official service provider for laravel to upload an image to Amazon S3. The image is temporarily stored on my server and should be deleted after uploading. The following is the code I used to do my upload and delete.

$temp_path = "/screenshot_temp/testing.png';

$client = AWS::createClient('s3');
$result = $client->putObject(array(
        'Bucket'     => self::$bucketName,
        'Key'        => 'screenshot/testing.png',
        'SourceFile' => $temp_path,
        'ACL'    => 'public-read'
    ));
);

chown($temp_path, 777);
unlink($temp_path);

The upload is successful. I can see my image with the link return, and I can see it on the amazon console. The problem is that the delete fails, with the following error message:

ErrorException: unlink(... path of my file ...): Permission denied

I am sure my file permission setting is correct, and I am able to delete my file with the section of code for uploading to S3 comment out. So it should be the problem that the file is locked during uploading the file. Is there a way I can unlock and delete my file?

via Chebli Mohamed