Write a function named split_stack that takes as a parameter an array stack of numbers,
and re-orders it so that all the non-negative numbers are at the top in the reverse of their original relative order,
and all the negative numbers are at the bottom in the reverse of their original relative order.
For example, if passed the stack [4, 0, -1, 5, -6, -3, 2, 7], your function should modify the stack to store [-3, -6, -1, 7, 2, 5, 0, 4].
Constraints: Do not declare any auxiliary data structures other than a single temporary array queue.
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) { ... }
