The Paragon Initiative site has posted a new guide to helping you integrate libsodium into your application to provide additional cryptographic functionality in addition to things like mcrypt and crypt

You shouldn't need a Ph.D in Applied Cryptography to build a secure web application. Enter libsodium, which allows developers to develop fast, secure, and reliable applications without needing to know what a stream cipher even is.

After reading this brief electronic manual, you should know what libsodium is, what features it has, and how to install it (both the library and the PHP extension from PECL). [You should also] generally understand which cryptography tool to use for a specific scenario [and] be capable of writing production-quality code that uses libsodium.

The guide (still a work in progress) starts by explaining what libsodium is and what it has to offer over other encryption methods. It talks about the role of random data in encryption, a few basic crypto concepts (like key-based encryption and hashing) and finally gets into some of the more advanced features of the libsodium extension.

Additionally, the guide is also open source so if you'd like to contribute, just submit a pull request for consideration.