How to remove error warning from WordPress site?

Question

PHP

Good afternoon! I changed the php version from 5.6 to 7.0 and now a warning appears on every page of the site:

Warning: Declaration of mainMenuWalker::start_el(&$output, $item, $depth, $args) should be compatible with Walker_Nav_Menu::start_el(&$output , $item, $depth = 0, $args = NULL, $id = 0) in /home/optmark/optmax.com.ua/www/wp-content/themes/optmaxtheme/functions.php on line 49

5 php version I can't use it because the plugin requires a fresh version of Php, and without it I can't edit pages.
Can you please tell me how to remove this warning from the site?

Answer 1

Removed the warning.
In the theme files in functions.php, I replaced the line start_el(&$output, $item, $depth, $args) with function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 )