There are several improvements that can be made to the functions you've given, based on my knowledge.
First, and most important, keep your private key secure, and make it external to the function definition (e.g., a second argument). I would recommend spending significant effort solving this problem. Is this per-user encryption, or application-wide encryption? If it's per-user, then why not use a key derivation algorithm, such as PBKDF2? If it's application-wide, then good luck with secure storage - I've thought about it before, and it seems non-trivial for a typical web app. Just make sure that you don't store the private key along with the source, nor (obviously) in the same database.
Also, as an aside: never use symmetric encryption for anything that's password-like. Use a one-way hash instead. I'm assuming, for this answer, that you need to actually recover the original CD key, rather than just verify a match.
Second, I'm dubious of the use of MCRYPT_RAND. It sounds like it might be the same PRNG as rand(), which is not ideal. Generating the IV with MCRYPT_DEV_URANDOM is what I'd recommend - /dev/random and /dev/urandom are both considered top-notch sources of cryptographic random numbers on Unix-based systems. Note that you don't generally want to use /dev/random for a web application, because it would open up an easy DoS vector - and /dev/urandom should suffice outside of embedded environments, anyway.
Other implementation issues
Now, with the major vulnerabilities that I can spot taken care of, a few more fine-grained details:
You're mixing OpenSSL and mcrypt functions. Also, openssl_encrypt is not documented in the official PHP docs (I suspect it's documented for C somewhere, though...). It might be better to use mcrypt_encrypt instead, since it's explicitly documented for PHP.
You're mixing CAST-256 and AES-256, as well. You should probably stick with AES-256 throughout, since that's the generally-advised standard algorithm for strong symmetric encryption. Note that AES-256 is MCRYPT_RIJNDAEL_128 used with a 256-bit key, and is NOT the same as Rijndael-256, which is something that I hadn't previously realized (the more you know!).
I'm not sure about the preferred cipher mode, but CBC is one that I have seen recommended for AES, at least. Hopefully someone else will have some input.