$price_unit = "20.45"
$qty = 'qty';
$net = 'net';
$net = $qty * $price_unit ;
as: 0<$qty<10 to parse in a textbox.
I do that :

if
($qty =0)
{
$net =0;
}

elseif
($qty =1) {
$net = $price_unit * 1;
}

elseif
($qty = 2) {
$net = $price_unit * 2;
}
....
else{echo "Message...."}

.....
<input type = "text" name = "qty" value = "<?php echo $qty ?>" size = "6">
<input type = "text" name = "gross" value = "$qty * $price_unit" size = "6">