If you use YAML in PHP, then you had until recently the choice between the C based PECL extension syck or the horrible spyc PHP based implementation. With syck it has the issue that it only supports the core standard and not stuff like merge-key, which is a very very useful extension to the core YAML standard.

He also points out another implementation that the Symfony framework has created - one built around regular expressions (slower, but a "more correct" version).

YAML, or YAML Ain't Markup Language, is a "human friendly data serialization standard". It's essentially a very basic format for storing data, and uses far less syntax than standard PHP. [...] It's almost like English; it's as basic as you want. Of course, that's not to say it doesn't support complex structures - this example demonstrates the power of the format.

He also points out the spyc extension that makes working with the files in PHP a breeze.

Recently I was working on developing an application that required a configuration file to setup. [...] XML is too hard to read for management-types. Plain text wasn't all that easy to handle from a code perspective. Then I remembered that when I played with Ruby on Rails it used something called YAML, and thought I'd give that go.

Searching around, he discovered the spyc library that aids in the YAML development and, as suggested by a commentor, the standards-compliant extension syck.