Update function.page.php

[alpaca-farm]

In PHP 8, line 112 may produce a Type Error: "Fatal error: Uncaught TypeError: Unsupported operand types: string - int" [see https://stackoverflow.com/questions/66238017/fatal-error-uncaught-typeerror-unsupported-operand-types-string-int-in]

To prevent this error, $current_page should be cast to int before the subtraction operation is done.