Update source/applications/database-references.txt

Author: Asparagirl

source/applications/database-references.txt

@@ -162,6 +162,10 @@ Support
    references manually or use the `MongoDBx::AutoDeref
    <http://search.cpan.org/dist/MongoDBx-AutoDeref/>`_ CPAN module.
 
+**PHP**
+   The PHP driver does support DBRefs, including the optional ``$db`` reference, through 
+   `The MongoDBRef class <http://www.php.net/manual/en/class.mongodbref.php/>`.
+
 **Python**
    The Python driver supports automatic resolution of DBRef
    relationships, and provides the :api:`DBref class <python/current/api/bson/dbref.html>`,