add with calc ???

[ranga]

I have the following problem. I want to read a value from a table , add some to it and then store the value back to the table. This is my code

$old = '{hts01_sinc_two___old}';
$in = '{hts01_sinc_two___in}';
$out = '{hts01_sinc_two___out}';
$new = $old + $in - $out;
return $new;

​

It does not work!
Can someone help me?

Thanks Ranga ...​