<?php

function deepFlatten($items)
{
    $result = [];
    foreach ($items as $item) {
        if (!is_array($item)) {
            $result[] = $item;
        } else {
            $result = array_merge($result, deepFlatten($item));
        }
    }

    return $result;
}

deepFlatten([1, [2], [[3], 4], 5]); // [1, 2, 3, 4, 5]