A Working Set is defined as a set of resources (projects, folders and files) that are joined together. They are used to group elements for display in views or for operations on the set.
If a working set is selected in the Package Explorer or Navigator, only the resources, children of resources, and parents of resources contained in the working set are shown. The Problems View, Tasks View and Bookmarks View can use working sets in their filters. When searching, a working set can be used to restrict the set of elements that are searched.
By default, the Window Working Set is used by default and is unfiltered.
You can place resources of any type into any Working Set regardless of their type (Java, PHP, etc.). A Working Set from within a PHP resource creates a PHP Working Set.
The name of the working set should be unique
While developing PHP applications in the PHP perspective, the Project Explorer view is your main view of your PHP projects and resources.
The Project Explorer view provides an integrated view of all project resources, including PHP resources, Java artifacts, resources, Web services, databases, and dynamic Web project artifacts.
Note: Newly created resources are not automatically included in the active working set. They are implicitly included in a working set if they are children of an existing working set element. If you want to include other resources after you have created them you have to explicitly add them to the working set.