Im 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});
                }
                else if(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. What would you do?