This function supports "base64url" as described in Section 5 of RFC 4648, "Base 64 Encoding with URL and Filename Safe Alphabet"

    <?php
    function base64url_decode($base64url)
    {
        $base64 = strtr($base64url, '-_', '+/');
        $plainText = base64_decode($base64);
        return ($plainText);
    }
    ?>