Variables are not interpolated in single-quoted strings. In fact, you don't need the strings at all. It's also a bit unclear whether you want to encode or decode; base64_encode() encodes a string in base64, base64_decode() decodes it.
And you're not seriously storing credit card numbers in this way, are you? No-one who manages to get into your server to look at the session data is going to be fooled by it.