flattenArray.php

<?php

function flattenArray($source, $key = null)
{
    $result = array();
    
    if ($key) {
        array_walk_recursive($source, function ($v, $k) use ($key, &$result) {
            if ($k === $key) {
                $result[] = $v;
            }
        });
    } else {
        array_walk_recursive($source, function ($v, $k) use (&$result) {
            $result[] = $v;
        });
    }

    return $result;
}