Write a function named mirror that accepts an array queue of strings as a parameter and appends the queue's contents to itself in reverse order.
For example, if an array queue named $q stores ["a", "b", "c"], the call of mirror($q); should change it to store ["a", "b", "c", "c", "b", "a"].
Constraints: You may declare a single array as auxiliary storage.
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) { ... }
