<?php
function array_flatten(array $array)
{
    return iterator_to_array(new RecursiveIteratorIterator(new RecursiveArrayIterator($array)), false);
}

/*
// Пример:
$a = [1, 2, [3, 4, 5, [6, 7], 8, 9], 10, 11];
print_r(array_flatten($a));
*/