Neo4jPHP by Josh Adell.
From the webpage:
PHP Wrapper for the Neo4j graph database REST interface
In-depth documentation and examples: http://github.com/jadell/neo4jphp/wiki
API documentation: http://jadell.github.com/neo4jphp
And from the introduction:
The goal of Neo4jPHP is to provide you with access to all the functionality of the Neo4j REST API via PHP. It does not provide a one-to-one correspondence with the REST API calls; instead, the REST interface is abstracted away so that you can concentrate on modelling your application’s domain in nodes and relationships. Neo4jPHP provides an API that is both intuitive and flexible, and it takes advantage of “under-the-hood” performance enhancements, such as caching and lazy-loading.
I just scanned through the documentation but it looks fairly clean and well-written. I pushed this out to the XML4Lib list but if you know of other library lists, please share it.