src/wp-admin/includes/menu.php

<?php
/**
 * Build Administration Menu.
 *
 * @package WordPress
 * @subpackage Administration
 */

if ( is_network_admin() ) {

	/**
	 * Fires before the administration menu loads in the Network Admin.
	 *
	 * The hook fires before menus and sub-menus are removed based on user privileges.
	 *
	 * @since 3.1.0
	 * @access private
	 */
	do_action( '_network_admin_menu' );
} elseif ( is_user_admin() ) {

	/**
	 * Fires before the administration menu loads in the User Admin.
	 *
	 * The hook fires before menus and sub-menus are removed based on user privileges.
	 *
	 * @since 3.1.0
	 * @access private
	 */
	do_action( '_user_admin_menu' );
} else {

	/**
	 * Fires before the administration menu loads in the admin.
	 *
	 * The hook fires before menus and sub-menus are removed based on user privileges.
	 *
	 * @since 2.2.0
	 * @access private
	 */
	do_action( '_admin_menu' );
}

// Create list of page plugin hook names.
foreach ( $menu as $menu_page ) {
	$pos = strpos( $menu_page[2], '?' );

	if ( false !== $pos ) {
		// Handle post_type=post|page|foo pages.
		$hook_name = substr( $menu_page[2], 0, $pos );
		$hook_args = substr( $menu_page[2], $pos + 1 );
		wp_parse_str( $hook_args, $hook_args );

		// Set the hook name to be the post type.
		if ( isset( $hook_args['post_type'] ) ) {
			$hook_name = $hook_args['post_type'];
		} else {
			$hook_name = basename( $hook_name, '.php' );
		}
		unset( $hook_args );
	} else {
		$hook_name = basename( $menu_page[2], '.php' );
	}

	$hook_name = sanitize_title( $hook_name );

	if ( isset( $compat[ $hook_name ] ) ) {
		$hook_name = $compat[ $hook_name ];
	} elseif ( ! $hook_name ) {
		continue;
	}

	$admin_page_hooks[ $menu_page[2] ] = $hook_name;
}
unset( $menu_page, $compat );

$_wp_submenu_nopriv = array();
$_wp_menu_nopriv    = array();
// Loop over submenus and remove pages for which the user does not have privs.
foreach ( $submenu as $parent => $sub ) {
	foreach ( $sub as $index => $data ) {
		if ( ! current_user_can( $data[1] ) ) {
			unset( $submenu[ $parent ][ $index ] );
			$_wp_submenu_nopriv[ $parent ][ $data[2] ] = true;
		}
	}
	unset( $index, $data );

	if ( empty( $submenu[ $parent ] ) ) {
		unset( $submenu[ $parent ] );
	}
}
unset( $sub, $parent );

/*
 * Loop over the top-level menu.
 * Menus for which the original parent is not accessible due to lack of privileges
 * will have the next submenu in line be assigned as the new menu parent.
 */
foreach ( $menu as $id => $data ) {
	if ( empty( $submenu[ $data[2] ] ) ) {
		continue;
	}

	$subs       = $submenu[ $data[2] ];
	$first_sub  = reset( $subs );
	$old_parent = $data[2];
	$new_parent = $first_sub[2];

	/*
	 * If the first submenu is not the same as the assigned parent,
	 * make the first submenu the new parent.
	 */
	if ( $new_parent !== $old_parent ) {
		$_wp_real_parent_file[ $old_parent ] = $new_parent;

		$menu[ $id ][2] = $new_parent;

		foreach ( $submenu[ $old_parent ] as $index => $data ) {
			$submenu[ $new_parent ][ $index ] = $submenu[ $old_parent ][ $index ];
			unset( $submenu[ $old_parent ][ $index ] );
		}
		unset( $submenu[ $old_parent ], $index );

		if ( isset( $_wp_submenu_nopriv[ $old_parent ] ) ) {
			$_wp_submenu_nopriv[ $new_parent ] = $_wp_submenu_nopriv[ $old_parent ];
		}
	}
}
unset( $id, $data, $subs, $first_sub, $old_parent, $new_parent );

if ( is_network_admin() ) {

	/**
	 * Fires before the administration menu loads in the Network Admin.
	 *
	 * @since 3.1.0
	 *
	 * @param string $context Empty context.
	 */
	do_action( 'network_admin_menu', '' );
} elseif ( is_user_admin() ) {

	/**
	 * Fires before the administration menu loads in the User Admin.
	 *
	 * @since 3.1.0
	 *
	 * @param string $context Empty context.
	 */
	do_action( 'user_admin_menu', '' );
} else {

	/**
	 * Fires before the administration menu loads in the admin.
	 *
	 * @since 1.5.0
	 *
	 * @param string $context Empty context.
	 */
	do_action( 'admin_menu', '' );
}

/*
 * Remove menus that have no accessible submenus and require privileges
 * that the user does not have. Run re-parent loop again.
 */
foreach ( $menu as $id => $data ) {
	if ( ! current_user_can( $data[1] ) ) {
		$_wp_menu_nopriv[ $data[2] ] = true;
	}

	/*
	 * If there is only one submenu and it is has same destination as the parent,
	 * remove the submenu.
	 */
	if ( ! empty( $submenu[ $data[2] ] ) && 1 === count( $submenu[ $data[2] ] ) ) {
		$subs      = $submenu[ $data[2] ];
		$first_sub = reset( $subs );

		if ( $data[2] === $first_sub[2] ) {
			unset( $submenu[ $data[2] ] );
		}
	}

	// If submenu is empty...
	if ( empty( $submenu[ $data[2] ] ) ) {
		// And user doesn't have privs, remove menu.
		if ( isset( $_wp_menu_nopriv[ $data[2] ] ) ) {
			unset( $menu[ $id ] );
		}
	}
}
unset( $id, $data, $subs, $first_sub );

/**
 * Adds a CSS class to a string.
 *
 * @since 2.7.0
 *
 * @param string $class_to_add The CSS class to add.
 * @param string $classes      The string to add the CSS class to.
 * @return string The string with the CSS class added.
 */
function add_cssclass( $class_to_add, $classes ) {
	if ( empty( $classes ) ) {
		return $class_to_add;
	}

	return $classes . ' ' . $class_to_add;
}

/**
 * Adds CSS classes for top-level administration menu items.
 *
 * The list of added classes includes `.menu-top-first` and `.menu-top-last`.
 *
 * @since 2.7.0
 *
 * @param array $menu The array of administration menu items.
 * @return array The array of administration menu items with the CSS classes added.
 */
function add_menu_classes( $menu ) {
	$first_item  = false;
	$last_order  = false;
	$items_count = count( $menu );

	$i = 0;

	foreach ( $menu as $order => $top ) {
		++$i;

		if ( 0 === $order ) { // Dashboard is always shown/single.
			$menu[0][4] = add_cssclass( 'menu-top-first', $top[4] );
			$last_order = 0;
			continue;
		}

		if ( str_starts_with( $top[2], 'separator' ) && false !== $last_order ) { // If separator.
			$first_item = true;
			$classes    = $menu[ $last_order ][4];

			$menu[ $last_order ][4] = add_cssclass( 'menu-top-last', $classes );
			continue;
		}

		if ( $first_item ) {
			$first_item = false;
			$classes    = $menu[ $order ][4];

			$menu[ $order ][4] = add_cssclass( 'menu-top-first', $classes );
		}

		if ( $i === $items_count ) { // Last item.
			$classes = $menu[ $order ][4];

			$menu[ $order ][4] = add_cssclass( 'menu-top-last', $classes );
		}

		$last_order = $order;
	}

	/**
	 * Filters administration menu array with classes added for top-level items.
	 *
	 * @since 2.7.0
	 *
	 * @param array $menu Associative array of administration menu items.
	 */
	return apply_filters( 'add_menu_classes', $menu );
}

uksort( $menu, 'strnatcasecmp' ); // Make it all pretty.

/**
 * Filters whether to enable custom ordering of the administration menu.
 *
 * See the {@see 'menu_order'} filter for reordering menu items.
 *
 * @since 2.8.0
 *
 * @param bool $custom Whether custom ordering is enabled. Default false.
 */
if ( apply_filters( 'custom_menu_order', false ) ) {
	$menu_order = array();

	foreach ( $menu as $menu_item ) {
		$menu_order[] = $menu_item[2];
	}
	unset( $menu_item );

	$default_menu_order = $menu_order;

	/**
	 * Filters the order of administration menu items.
	 *
	 * A truthy value must first be passed to the {@see 'custom_menu_order'} filter
	 * for this filter to work. Use the following to enable custom menu ordering:
	 *
	 *     add_filter( 'custom_menu_order', '__return_true' );
	 *
	 * @since 2.8.0
	 *
	 * @param array $menu_order An ordered array of menu items.
	 */
	$menu_order = apply_filters( 'menu_order', $menu_order );
	$menu_order = array_flip( $menu_order );

	$default_menu_order = array_flip( $default_menu_order );

	/**
	 * @global array $menu_order
	 * @global array $default_menu_order
	 *
	 * @param array $a
	 * @param array $b
	 * @return int
	 */
	function sort_menu( $a, $b ) {
		global $menu_order, $default_menu_order;

		$a = $a[2];
		$b = $b[2];

		if ( isset( $menu_order[ $a ] ) && ! isset( $menu_order[ $b ] ) ) {
			return -1;
		} elseif ( ! isset( $menu_order[ $a ] ) && isset( $menu_order[ $b ] ) ) {
			return 1;
		} elseif ( isset( $menu_order[ $a ] ) && isset( $menu_order[ $b ] ) ) {
			if ( $menu_order[ $a ] === $menu_order[ $b ] ) {
				return 0;
			}
			return ( $menu_order[ $a ] < $menu_order[ $b ] ) ? -1 : 1;
		} else {
			return ( $default_menu_order[ $a ] <= $default_menu_order[ $b ] ) ? -1 : 1;
		}
	}

	usort( $menu, 'sort_menu' );
	unset( $menu_order, $default_menu_order );
}

// Prevent adjacent separators.
$prev_menu_was_separator = false;
foreach ( $menu as $id => $data ) {
	if ( false === stristr( $data[4], 'wp-menu-separator' ) ) {

		// This item is not a separator, so falsey the toggler and do nothing.
		$prev_menu_was_separator = false;
	} else {

		// The previous item was a separator, so unset this one.
		if ( true === $prev_menu_was_separator ) {
			unset( $menu[ $id ] );
		}

		// This item is a separator, so truthy the toggler and move on.
		$prev_menu_was_separator = true;
	}
}
unset( $id, $data, $prev_menu_was_separator );

// Remove the last menu item if it is a separator.
$last_menu_key = array_keys( $menu );
$last_menu_key = array_pop( $last_menu_key );
if ( ! empty( $menu ) && 'wp-menu-separator' === $menu[ $last_menu_key ][4] ) {
	unset( $menu[ $last_menu_key ] );
}
unset( $last_menu_key );

if ( ! user_can_access_admin_page() ) {

	/**
	 * Fires when access to an admin page is denied.
	 *
	 * @since 2.5.0
	 */
	do_action( 'admin_page_access_denied' );

	wp_die( __( 'Sorry, you are not allowed to access this page.' ), 403 );
}

$menu = add_menu_classes( $menu );