In PHP, arrays are actually "ordered maps" that are used to contain a collection of items. When designing your own data structures, if an object represents a collection of items it should be array-accessible, using the most appropriate interface:
ArrayAccessfor collections that are referred to by indexIteratorfor collections that will be used within loops
Not all types of collection should be both an ArrayAccess and an Iterator. For example, imagine a class that represents a large dataset from a database. It may make sense to only implement the Iterator functions, so the data can be streamed rather than all loaded into memory at once.
When designing your own data structures, if an object represents a single item, its data attributes should be accessed via getter functions, rather than as an array-like object, or as public properties where acceptable.
Public properties should be used sparingly, and when they are needed it is recommended to consider read-only properties or magic methods to provide access to an object's internal data, rather than exposing properties directly. This allows for exposing read-only properties as well as improving maintainability when code is refactored in the future.
When getter functions are used on objects with unknown/arbitrary data contents, such as a database row, the getters should be provided using a type safe interface.
Storing data within a native array in PHP should only be done when dealing with simple key-value-pairs, and should always be wrapped by an object's getters/setters. This allows for improving and refactoring the functionality of an object without having backwards breaking side effects.
It's possible to union two arrays with the +, and _push an item into an array with [] =, but this style of coding has low readability. Unless a union is expected, use the array_merge function, and always use array_push or array_unshift to add an item into an array (even if there's only one item being added).