<?php

// Brief:
// Write some code, that will flatten an array of arbitrarily
// nested arrays of integers into a flat array of integers.
// e.g. [[1,2,[3]],4] -> [1,2,3,4].

// This code requires at least PHP 5.4.0 to run.

// Test input.
$input = [[13,77],[1,2,[3,5,[8,6]]],4];
// $input = [2];
// $input = [1,2];
// Non-array input should cause an exception to be thrown.
// $input = 4;

$output = [];

flatten_array($input, $output);

// json_encode() just so the output looks nice.
echo "\n INPUT:   " . json_encode($input) . "\n";
echo "\n OUTPUT:  " . json_encode($output) . "\n\n";

/**
 * Recursively traverse an array, adding non-array values to a second array.
 *
 * @param array $input
 *   A multi-dimensional array of values to flatten.
 * @param array $output
 *   An array used to store the found values.
 *   Passed by reference so its state can be maintained.
 *
 * @return array
 *   The input array as a flat array.
 */
function flatten_array(array $input, array &$output) {
  foreach ($input as $value) {
    if (is_array($value)) {
      $output += flatten_array($value, $output);
    }
    else {
      $output[] = $value;
    }
  }
  return $output;
}