...But we use it for everything, because PHP is pragmatic: “dealing with things sensibly and realistically in a way that is based on practical rather than theoretical considerations”. An array gets the job done. Unfortunately, with flexibility comes complexity. Collection is the base interface which covers common functionality like foreach, echo, count, print_r, var_dump, serialize, json_encode, and clone. Sequence describes the behavior of values arranged in a single, linear dimension. Some languages refer to this as a List. Documentation is available on php.net. You should also include the polyfill in your project for IDE integration.