Francois Zaninotto submitted a tutorial he's written up about creating a YAML validation script with PHP.
As of today, there is no simple way to validate the syntax of a YAML file in PHP. But with two simple tricks, it takes only a few dozens of lines of code to build a robust validator capable of checking the syntax of any YAML file against a given schema.
He points out that Ruby has a tool for this (kwalify) but PHP doesn't. He creates his own with the help of the sfYaml component from the symfony framework, translating the YAML data into something PHP can parse more easily - XML. He passes this through an XSL parser and uses the DOM XML schemaValidate function to check it against the given schema.