Converting keys of an array/object-tree to lowercase

I am currently optimizing a PHP application and found one function being called around 10-20k times, so I'd thought I'd start optimization there:

function keysToLower($obj)
{
    if (!is_object($obj) && !is_array($obj))
        return $obj;

    foreach ($obj as $key => $element) {
        $element = keysToLower($element);
        if (is_object($obj)) {
            $obj->{strtolower($key)} = $element;
            if (!ctype_lower($key))
                unset($obj->{$key});
        } elseif (is_array($obj) && ctype_upper($key)) {
           $obj[strtolower($key)] = $element;
           unset($obj[$key]);
        }
    }
    return $obj;
}

Most of the time is spent in recursive calls (which are quite slow in PHP), but I don't see any way to convert it to a loop. How can I do this?

Foreach is using an internal copy that is then traversed. Try it without:

function keysToLower($obj)
{
    $type = (int) is_object($obj) - (int) is_array($obj);
    if ($type === 0) return $obj;
    reset($obj);
    while (($key = key($obj)) !== null)
    {
        $element = keysToLower(current($obj));
        switch ($type)
        {
        case 1:
            if (!is_int($key) && $key !== ($keyLowercase = strtolower($key)))
            {
                unset($obj->{$key});
                $key = $keyLowercase;
            }
            $obj->{$key} = $element;
            break;
        case -1:
            if (!is_int($key) && $key !== ($keyLowercase = strtolower($key)))
            {
                unset($obj[$key]);
                $key = $keyLowercase;
            }
            $obj[$key] = $element;
            break;
        }
        next($obj);
    }
    return $obj;
}

Or use references to avoid that a copy is used:

function &keysToLower(&$obj)
{
    $type = (int) is_object($obj) - (int) is_array($obj);
    if ($type === 0) return $obj;
    foreach ($obj as $key => &$val)
    {
        $element = keysToLower($val);
        switch ($type)
        {
        case 1:
            if (!is_int($key) && $key !== ($keyLowercase = strtolower($key)))
            {
                unset($obj->{$key});
                $key = $keyLowercase;
            }
            $obj->{$key} = $element;
            break;
        case -1:
            if (!is_int($key) && $key !== ($keyLowercase = strtolower($key)))
            {
                unset($obj[$key]);
                $key = $keyLowercase;
            }
            $obj[$key] = $element;
            break;
        }
    }
    return $obj;
}