Service Data Objects (SDOs) enable PHP applications to work with data from different sources (like a database query, an XML file, and a spreadsheet) using a single interface.
Each different kind of data source requires a Data Access Service (DAS) to provide access to the data in the data source. In your PHP application, you use a DAS to create an SDO instance that represents some data in the data source. You can then set and get values in the SDO instance using the standard SDO interface. Finally, you use a DAS to write the modified data back to a data source, typically the same one.
See the list of Data Access Services for details on those currently available. In addition to the provided DASs, SDO also provides interfaces to enable others to be implemented (see the section on SDO Data Access Services Interface for more details).
A Service Data Object instance is made up of a tree of data objects. The tree is defined by containment relationships between the data objects. For example, a Company data object might consist of a number of Department data objects and therefore the Company would have a containment relationship to the Departments.
An SDO may also have non-containment references between data objects in the tree. For example, one Employee data object might reference another Employee to identify a career mentor.
As well as data objects referencing each other, they can also have primitive properties. For example, the Company data object might have a property called "name" of type string, for holding the name of the company (for example, "Acme").
Each of these properties of a data object - containment relationships, non-containment references, or primitive properties - may be many-valued or single-valued. In the above examples, Departments is many-valued and the Company name is single-valued.
In PHP, each SDO data object is represented as a PHP object. The properties of the data object can be accessed using either object syntax or associative array syntax. We'll see some examples of this later.