How to add a custom HTML class name to admin screen submenu items?

You can do a str_replace on the html which will happen before the page has loaded:

Just need to work out the selectors or then parse it with DOMDocument

function callback($buffer) {    $buffer = str_replace('wp-first-item', 'wp-first-item my-custom-class', $buffer);    return $buffer;}function buffer_start() { ob_start("callback"); }function buffer_end() { ob_end_flush(); }add_action('admin_head', 'buffer_start');add_action('admin_footer', 'buffer_end');

We can actually do it with a simple plugin like this one:

<?php/** Plugin Name: Custom Admin Submenu CSS Class **/add_action( 'admin_menu', function() use ( &$submenu ){    $class = 'my-class'; // Edit to your needs!    if( ! isset( $submenu['edit.php'][5] ) )        return;    if( ! empty( $submenu['edit.php'][5][4] ) ) // Append if css class exists        $submenu['edit.php'][5][4] .= ' ' . $class;    else                                              $submenu['edit.php'][5][4] = $class;} );

We constructed it this way by spotting out this this part of the _wp_menu_output() core function:

if ( ! empty( $sub_item[4] ) ) {    $class[] = esc_attr( $sub_item[4] );}

Here's how the modified HTML looks like:

<ul class='wp-submenu wp-submenu-wrap'>    <li class='wp-submenu-head' aria-hidden='true'>Posts</li>    <li class="wp-first-item current my-class">        <a href='edit.php' class="wp-first-item current my-class">All Posts</a>    </li>    <li>        <a href='post-new.php'>Add New</a>    </li>    <li>        <a href='edit-tags.php?taxonomy=category'>Categories</a>    </li>    <li>        <a href='edit-tags.php?taxonomy=post_tag'>Tags</a>    </li></ul>

where the custom css class is added to both the <li> tag and the <a> tag.

In general I don't like modifying a global variable, but there doesn't seems to be a workaround to add the class to the submenu via add_submenu_page() or other explicit filters.

If you want to modify the css classes for the first level items (menu), you can e.g. look into the add_menu_classes filter.

If you feel strongly that the css classes for the submenus should be directly adjustable via a filter, then you can create a trac ticket, explain in detail why this is needed and e.g. suggest a new add_submenu_classes filter.

I hope it helps!

This should do it:

function add_admin_class() {    $find = '.wp-submenu li';    $add_class = 'my-custom-class';    echo '"<script type="text/javascript">        jQuery(function() {            jQuery("' . $find . '").addClass("' . $add_class . '");        });    </script>"';}add_action('admin_footer', 'add_admin_class');