Windows 7: PHP/RSA/open_ssl help

I know it's not Windows 7 topic, but...
I have a .NET client app that generates RSA key (private/public pair). Public key is Base64-ed and sent as text to a linux server with PHP 4.3.11 running on it. I need php code to encrypt some arbitrary string (can be small one, I can do chunking myself) and return it back to the client in a text form (i.e. Base64-ed again), so it can decode it with its private key.
There are a couple of the open_ssl functions that look like they can do the trick, but since I don't know PHP I can't get my mind to wrap around how shall I pass the public key string into them...
I also tried Crypt_RSA lib with about same results..
Any help appreciated!