try changing:
$PP_DirectPayment_optional[0][$nextIndex] = "SH_Name";
$PP_DirectPayment_optional[1][$nextIndex] = "".((isset($_POST["shipping_firstname"]))?$_POST["shipping_firstname"]:"") ."";
to:
$PP_DirectPayment_optional[0][$nextIndex] = "SH_Name";
$PP_DirectPayment_optional[1][$nextIndex] = "".((isset($_POST["shipping_firstname"]))?$_POST["shipping_firstname"]:"") ." ".((isset($_POST["shipping_lastname"]))?$_POST["shipping_lastname"]:"") ."";