The function fills an array with num entries of the value of the array_values parameter, keys starting at the starting_index parameter. Increment of Values. 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. If an associative array is used as the second parameter of array_fill_keys, then the associative array will be appended in all the values of the first array.