I slightly fiddled with the Walker_Nav_Menu class in wordpress, the goal was to replace the standard menu classes with my own.
Here is the code:

function start_el (& amp; $ output, $ item, $ depth, $ args) {
  // here I add my values ​​to the array with classes.
  // assign classes to the li element and display it
  $ item- & gt; classes ['my_item_class'] = 'page-header__nav_item'; // class of li element
  $ item- & gt; classes ['my_link_class'] = 'page-header__nav_link'; // class a of the element
  if ($ item- & gt; current) {
    $ item- & gt; classes ['my_current_class'] = 'page-header__nav_link - active'; // class a of the element of the current menu item
  }
  $ class_names = 'class = "' .esc_attr ($ item- & gt; classes ['my_item_class']. ''. $ item- & gt; classes ['parent_class']). '"';
  $ output. = '& lt; li'. $ class_names. '& gt;';
  // assign attributes to a-element
  $ attributes. =! empty ($ item- & gt; url)? 'href = "' .esc_attr ($ item- & gt; url). '" class = "'. $ item- & gt; classes ['my_link_class']. ''. $ item- & gt; classes ['my_current_class']. '"': '';
  $ item_output = $ args- & gt; before;

array with classes $ item- & gt; classes , after these manipulations it looks like:

Array
(
  [0] = & gt;
  [1] = & gt; menu-item
  [2] = & gt; menu-item-type-custom
  [3] = & gt; menu-item-object-custom
  [4] = & gt; current-menu-item
  [5] = & gt; current_page_item
  [6] = & gt; menu-item-home
  [my_item_class] = & gt; page-header__nav_item
  [my_link_class] = & gt; page-header__nav_link
  [my_current_class] = & gt; page-header__nav_link - active
)

An error occurs:

Warning: Illegal string offset ‘my_item_class’ in
/var/www/vhosts/u0205156.plsk.regruhosting.ru/blog.itconcentrate.pw/wp-content/themes/gragon/functions.php
on line 221

Well, and, accordingly, for all the elements of the classes array added by me.
What’s the problem?

Answer 1, authority 100%

This should fix the problem. As I understand it, the problem is related to the PHP version.

if (! isset ($ item- & gt; classes ['my_item_class'])) // fix
 {
  $ item- & gt; classes ['my_item_class'] = 'page-header__nav_item'; // class li
 }