Wordpress Theme Errors - PHP Upgrade

A client of ours has some wordpress site errors since upgrading from PHP 5.8 to 7.2

As a result they get lots of errors and the site does not load:

Errors include:

Warning: Declaration of Portfolio_Walker::start_el(&$output, $category, $depth, $args) should be compatible with Walker_Category::start_el(&$output, $category, $depth = 0, $args = Array, $id = 0) in

Header information - headers already sent by (output started at

I need someone to fix ALL the errors on the site - its a small job