The DevX.com site has posted an introductory guide to using cryptography in PHP, showing how to use the various packages the language has to offer.

Cryptography is just one piece of the security puzzle, along with SSL/TLS, certificates, digital signatures, and so on. This article explains how to use PHP to implement the most common cryptographic algorithms. In addition to describing PHP's default encryption functions, you'll see how to use a wide variety of cryptographic libraries and packages.

They start with a look at some of the built-in functions like md5, sh1 and crypt as well as a table detailing the different encryption methods (like mcrypt, mhash or crypt_blowfish). They follow this up with examples of some of them including a method for making secret keys with the Crypt_DiffieHellman PEAR Package.

Harry Fuecks came across a need in his development work to bridge a gap between a PHP script and some Java work he'd done. He found the way that fit his situation the best - the encryption of the data on the PHP side via OpenSSL.

Needed a mechanism to be able to pass chunks of data securely from PHP to Java [...] One solution might be something "from scratch" involving mcrypt or PHP libraries like Crypt_RSA. [...] Another approach is GnuPG, either via the command line as discussed in this tutorial or via the GnuPG extension from PECL.

Option 3 is using OpenSSL and PHP's openssl_seal() function. SSL is normally used for encrypting networked communication between peers but that's not all it can do. [...] What's more - and perhaps the biggest win - it also allows us to re-use existing SSL certificates.

He uses the openssl_seal functionality on top of the EVP encrypted envelope on the certificate to handle the encrypt/decrypt of the data. Both the PHP code and Java code (and execution example) are included in the post.