An array in PHP is actually an ordered map. A map is a type that
associates values to keys. This type
is optimized for several different uses; it can be treated as an array,
list (vector), hash table (an implementation of a map), dictionary,
collection, stack, queue, and probably more. As array values can
be other arrays, trees and multidimensional arrays
are also possible.
Explanation of those data structures is beyond the scope of this manual, but
at least one example is provided for each of them. For more information, look
towards the considerable literature that exists about this broad topic.
