I first came across PHPCR through my work with Magnolia CMS, an open source Java CMS. Magnolia CMS uses the Java Content Repository (JSR-283) as its content storage mechanism and PHPCR is a PHP adaptation of that standard. It’s great for storing semi-structured content in a hierarchical format and it also includes support for content versioning and indexing.
The cool thing about PHPCR is that it lets PHP developers interact with content stored in a JCR-compliant content repository using PHP objects and APIs…no Java knowledge needed. This means that you could save data to a JCR repository using a Java application, then read or modify that data using PHP, simplifying data reuse and interoperability between applications running on different platforms.
Read more in my tutorial on integrating PHP applications with Magnolia CMS. It discusses a number of common use cases, including reading and writing content to a JCR repository and integrating a PHP front-end with a Java application via the JCR.