if you only need to query the server every 30 seconds or so, i see no need of having a persistent connection.

If you do go with an actual server instead of php scripts, I also see no point of having memory cache (assuming we are still talking about a small turn based game). If profiling proves me wrong, this optimization can be done at a later stage once you have your prototype up and running.

(oracle has a hard limit of about 1,000 transactions per second as far as i remember)

If you want to send data safe use something well proven like diffie hellmann.
http://en.wikipedia.org/wiki/Diffie-Hellman_key_exchange

i guess google will turn up a few written code examples both for most programming languages.

Quote:diffie hellmann

It turns out that diffie-hellman isn't secure against man-in-the-middle attacks, because the man in the middle can run a separate exchange towards each end, and decrypt/convert in the middle. You need some way of authenticating the sender and receiver, which means that you need either a shared secret or public-key cryptography, which means that Diffie-Hellman isn't actually needed in the end.