Add separators to WP Admin menus

insert_menu_separator.php

<?php

namespace WCM;

// Add the filter
add_action( 'admin_menu', __NAMESPACE__.'\add_menu_separator' );
function add_menu_separator()
{
	add_filter( 'add_menu_classes', __NAMESPACE__.'\insert_admin_menu_separator' );
}

/**
 * Adds separators for menu item groups
 * 
 * @param (array) $menu
 */
function insert_admin_menu_separator( $menu )
{
	// Targeted elements
	$targets = array( 
		'Books' 
	);

	// Need the keys as numeric array for easier look up
	$index = array_keys( $menu );

	foreach ( $targets as $target )
	{
		foreach ( $menu as $m_pos => $m )
		{
			foreach ( $m as $key => $data )
			{
				// Get index from keys array - strict to speed things up
				if ( $target === $data )
					$pos_first = array_search( $m_pos, $index, true );
			}
		}
		// Get menu position from current, previous & next element
		$prev_el = $index[ $pos_first - 1 ];
		$next_el = $index[ $pos_first + 1 ];
		$curr_el = $index[ $pos_first ];

		// Check if (exactly) previous key has no separator attached
		// and is not set (else we'd overwrite the element)
		if ( ! isset( $menu[ $curr_el - 1 ] ) AND $menu[ $curr_el - 1 ] !== 'wp-menu-separator' )
		{
			$separator = array( 
				0 => '',
				1 => 'read',
				2 => 'separator',
				3 => '',
				4 => 'wp-menu-separator',
			);
			// insert separator before the targeted element (first in group)
			$menu[ $curr_el - 1 ] = $separator;
		}
		elseif ( isset( $menu[ $curr_el - 1 ] ) )
		{
			# @todo Move index around
		}

		// Add css classes to previous element
		$prev = $menu[ $prev_el ][4];
		$menu[ $prev_el ][4] = add_cssclass( 'menu-top-last', $prev );

		// Add css classes to next element
		$next = $menu[ $next_el ][4];
		$menu[ $next_el ][4] = add_cssclass( 'menu-top-first', $next );
	}

	// sort
	uksort( $menu, 'strnatcasecmp' );

	return $menu;
}