It's important to recognize that in PHP an array does not always behave like a traditional array. It's actually more representative of an ordered hashmap. Thus it maintains order, but consists of key/value pairs. A key, in a PHP array, is always unique, but can be either an integer or a string. If the string key can be cast to an integer then PHP will carry out this operation implicitly. Also, array keys do not determine the order of elements in a PHP array.
With this in mind, we can better understand how different operations, like diff, intersect, union, and merge, would be carried out on a native PHP array. Some of these operations rely on the array keys and others on the values in the array.