A resource is a special variable, holding a reference to an external resource. Resources are created and used by special functions. See the appendix for a listing of all these functions and the corresponding resource types.
The resource type was introduced in PHP 4
See also get_resource_type().
As resource types hold special handlers to opened files, database connections, image canvas areas and the like, you cannot convert any value to a resource.
Due to the reference-counting system introduced with PHP 4's Zend Engine, it is automatically detected when a resource is no longer referred to (just like Java). When this is the case, all resources that were in use for this resource are made free by the garbage collector. For this reason, it is rarely ever necessary to free the memory manually by using some free_result function.
Persistent database links are special, they are not destroyed by the garbage collector. See also the section about persistent connections.