Alexey Zakhlestin mentions some new YAML functionality he's worked up - emitter functionality:

One more good news for you, YAML-users. I've implemented emitter functionality in php-binding of syck library. It is quite straight-forward, no fancy stuff for now, but it should work.

Supported datatypes include associative arrays, strings, integers, floats, booleans and nulls. He includes an example of the code in action (dumping out the information for an associative array). You can get this latest update from the subversion source on his site.