Let's assume you intended this thread to be only about arrays of integer variables.
Note that "minimum" and "maximum" may be undefined (or at least ambiguous) depending on the size of the array and the distribution of data it holds. Hence josh's desire to add parameters to the function to gain at least some minimal information about the data you are going to give the function in order to produce the statistic.
What is the maximum value of an empty array?
Given the array [5, 5, 5], what is the index of its maximum value?
For sorted arrays, the built-in Low() and High() functions do give you the indices of the minimum and maximum values. Well, strictly, the first index of a minimum value and the last index of a maximum value, when the sorted array contains duplicates (and is sorted in ascending order).
This is obviously faster than iterating the entire array looking for a maximum/minimum value (but only because array iterations have already taken place to produce the sorted array).