The code you showed passes a string of numeric digits, a single-digit integer, and a string of characters held in $chars to a funtion named num2code, capturing the output into $code. You didn't show the line that actually produces output.

I can look at the output and formulate a theory as to what num2code does, but why don't you look at the documentation for num2code, or the source code for the function, and then follow up here if you still have a question.