Write a function named mirror that accepts an array of strings as a parameter and produces a mirrored
copy of the array as output, with the original values followed by those same values in the opposite order.
For example, if an array variable named $arr contains the values
["a", "b", "c"], after a call of mirror($arr) it should contain
["a", "b", "c", "c", "b", "a"].
If there are an odd number of values, the last element is not moved.
You may assume that the array is not null and that no element of the array is null.
A note about references in PHP: In order to write a function passes a parameter as
reference (thus modifying its state), you'll need to prepend "&" to the variable declaration
in the function header. For example, a function foo that modifies the state
of an array parameter may be defined as:
function foo(&$arr) { ... }
