I\'m trying to test this example that I found here so that I can do a direct upload on the client side without having the user login using Google Cloud Storage.
First, you need to use openssl_pkcs12_read to read the key file, not file_get_contents. Second, I believe you want to leave off the second parameter to openssl_get_privatekey.
I highly recommend you use google-api-php-client for this, which has Google_P12Signer.php