Update ArrayStack.php

Array object cannot be defined twice, this is a minor typo

stack call:
Fatal error: Cannot use ArrayObject as ArrayObject because the name is already in use
Merge pull request #3984 from smit1562/patch-1

Update ArrayStack.php
[#3984] Import ArrayObject as PhpArrayObject

- and update class declaration to extend PhpArrayObject
Merge branch 'hotfix/3984'

Close #3984