PHP - Have Never Seen += Operand

I'm getting this error for a blog script that is on my site :-

PHP Fatal error:  Uncaught TypeError: Unsupported operand types: string - int

The line in questions is this :-

$prev = $page - 1;

If I comment out this block of code the blog appears (with other errors, but I'll move on to those if I can fix this first!).

/* Setup page vars for display. */
    if (isset($page) and $page == 0) $page = 1;                    //if no page var is given, default to 1.
    $prev = $page - 1;                            //previous page is page - 1
    $next = $page + 1;                            //next page is page + 1
    $lastpage = ceil($total_pages/$limit);        //lastpage is = total pages / items per page, rounded up.
    $lpm1 = $lastpage - 1;                        //last page minus 1

Any idea's on what's needed to fix this script? I've contacted the original author but they haven't got back to me, I guess PHP8 maybe a little too new for them.

Thanks for any help....

In PHP Version 8.0 shows error as :
PHP Fatal error:  Uncaught TypeError: Unsupported operand types: string * string 
String is as under :
$TAXVALUE= round($QNTY*($PRICE- ($PRICE*$DISC/100)),2);

In previous versions of PHP it does not show any error.

Please resolve the issue.