Spyc: https://github.com/mustangostang/spyc
Pure PHP implementation, so you don't need to make any modifications to the server for installation. If speed is of dire concern, it might not be the ideal solution, but if you're using YAML for configurations or relatively low-volume use, it is a fantastic solution.
Given a YAML document, Spyc will return an array that you can use however you see fit.
require_once "spyc.php";
$data = Spyc::YAMLLoad($myfile);
Given an array, Spyc will return a string which contains a YAML document built from your data.
$yaml_str = Spyc::YAMLDump($myarray);