The code kept giving me an error. I changed it to this but I'm not sure if it is correct. This doesn't give me an error.
$PP_DirectPayment_optional[0][$nextIndex] = "SH_Name";
$PP_DirectPayment_optional[1][$nextIndex] = "".((isset($_POST["shipping_firstname"]))?$_POST["shipping_firstname"]:"") ." ".((isset($_POST["shipping_lastname"]))?$_POST["shipping_lastname"]:"") ."";