Iterative approach to flatten a multi-dimensional array of values.

flatten_array.php

<?php
namespace FlattenArray;

/**
* Iterative approach to flatten a multi-dimensional array of values. 
* Notes: Non-array values are untouched so this could be an array of integers, strings, objects etc. An iterative approach is 
* taken here over a recursive one as to not overload the call stack on deeply nested arrays.
*/

function flatten($ar) {
    $newList = [];
    
    while (count($ar) > 0) {
        $item = array_shift($ar);
        
        // If this element is an array, push its values onto the front of our current list
        if (is_array($item)) {
            $ar = array_merge($item, $ar);
        }
        else {
            // If not an array, push it onto the final value list
            array_push($newList, $item);
        }
    }
    
    return $newList;
}