I need a get order by "Author" added to this PHP section,

I need a get order by "Author" added to this PHP section, help or a quote would be appreciated!

template-functions.php at line 1319
//get order by
if (!$order_by) {
if ($mp->get_setting('order_by') == 'price')
$order_by_query = '&meta_key=mp_price_sort&orderby=meta_value_num';
else if ($mp->get_setting('order_by') == 'sales')
$order_by_query = '&meta_key=mp_sales_count&orderby=meta_value_num';
else
$order_by_query = '&orderby='.$mp->get_setting('order_by');
} else {
if ('price' == $order_by)
$order_by_query = '&meta_key=mp_price_sort&orderby=meta_value_num';
else if('sales' == $order_by)
$order_by_query = '&meta_key=mp_sales_count&orderby=meta_value_num';
else
$order_by_query = '&orderby='.$order_by;
}