[WordPress] Highlight an Active Submenu in the Back-End

00-highlight-custom-submenu.php

<?php

add_filter( 'parent_file', 'acme_highlight_draft_submenu' );
/**
 * Highlights the 'Draft' submenu item using WordPress native styles.
 *
 * @param string $parent_file The filename of the parent menu.
 *
 * @return string $parent_file The filename of the parent menu.
 */
function acme_highlight_draft_submenu( $parent_file ) {

	global $submenu_file;

	// Only update the submenu file value if we're on the Draft page.
	if ( 'draft' === get_query_var( 'post_status' ) ) {

    		$args = array( 'post_status' => 'draft', );

		// Use the full URL so get the admin URL and concatenate it with the parent file.
		$admin_url = trailingslashit( get_admin_url() );
		$url = sprintf( '%s%s', $admin_url, $parent_file );

		$submenu_file = add_query_arg( $args, $url );
	}

	return $parent_file;
}