Josh Adell has written up a new post for his blog showing some of his experience connecting PHP to Neo4j, a graph database (and open source project). It helps to solve a problem of related datasets and the difficulties that traditional RDBMS have with them.
Lately, I've been playing around with the graph database Neo4j and its application to certain classes of problems. Graph databases are meant to solve problems in domains where data relationships can be multiple levels deep. [...] As you continue to add degrees, the query becomes increasingly unwieldy, harder to maintain, and less performant.
Graph databases, like Neo4j, handle these sorts of needs perfectly. There wasn't an official PHP interface for it, so he decided to create his own that worked with the REST interface the database provides. He includes a few code snippets showing how to connect and make a few requests - setting properties on objects, relating objects to one another and making queries that follow the paths from user to user and return them as a simple array.