Source

File: wp-admin/includes/plugin.php

function add_submenu_page( $parent_slug, $page_title, $menu_title, $capability, $menu_slug, $function = '', $position = null ) {
	global $submenu, $menu, $_wp_real_parent_file, $_wp_submenu_nopriv,
		$_registered_pages, $_parent_pages;

	$menu_slug   = plugin_basename( $menu_slug );
	$parent_slug = plugin_basename( $parent_slug );

	if ( isset( $_wp_real_parent_file[ $parent_slug ] ) ) {
		$parent_slug = $_wp_real_parent_file[ $parent_slug ];
	}

	if ( ! current_user_can( $capability ) ) {
		$_wp_submenu_nopriv[ $parent_slug ][ $menu_slug ] = true;
		return false;
	}

	/*
	 * If the parent doesn't already have a submenu, add a link to the parent
	 * as the first item in the submenu. If the submenu file is the same as the
	 * parent file someone is trying to link back to the parent manually. In
	 * this case, don't automatically add a link back to avoid duplication.
	 */
	if ( ! isset( $submenu[ $parent_slug ] ) && $menu_slug != $parent_slug ) {
		foreach ( (array) $menu as $parent_menu ) {
			if ( $parent_menu[2] == $parent_slug && current_user_can( $parent_menu[1] ) ) {
				$submenu[ $parent_slug ][] = array_slice( $parent_menu, 0, 4 );
			}
		}
	}

	$new_sub_menu = array( $menu_title, $capability, $menu_slug, $page_title );
	if ( ! is_int( $position ) ) {
		if ( null !== $position ) {
			_doing_it_wrong(
				__FUNCTION__,
				sprintf(
					/* translators: %s: add_submenu_page() */
					__( 'The seventh parameter passed to %s should be an integer representing menu position.' ),
					'<code>add_submenu_page()</code>'
				),
				'5.3.0'
			);
		}

		$submenu[ $parent_slug ][] = $new_sub_menu;
	} else {
		// Append the submenu if the parent item is not present in the submenu,
		// or if position is equal or higher than the number of items in the array.
		if ( ! isset( $submenu[ $parent_slug ] ) || $position >= count( $submenu[ $parent_slug ] ) ) {
			$submenu[ $parent_slug ][] = $new_sub_menu;
		} else {
			// Test for a negative position.
			$position = max( $position, 0 );
			if ( 0 === $position ) {
				// For negative or `0` positions, prepend the submenu.
				array_unshift( $submenu[ $parent_slug ], $new_sub_menu );
			} else {
				// Grab all of the items before the insertion point.
				$before_items = array_slice( $submenu[ $parent_slug ], 0, $position, true );
				// Grab all of the items after the insertion point.
				$after_items = array_slice( $submenu[ $parent_slug ], $position, null, true );
				// Add the new item.
				$before_items[] = $new_sub_menu;
				// Merge the items.
				$submenu[ $parent_slug ] = array_merge( $before_items, $after_items );
			}
		}
	}
	// Sort the parent array.
	ksort( $submenu[ $parent_slug ] );

	$hookname = get_plugin_page_hookname( $menu_slug, $parent_slug );
	if ( ! empty( $function ) && ! empty( $hookname ) ) {
		add_action( $hookname, $function );
	}

	$_registered_pages[ $hookname ] = true;

	/*
	 * Backward-compatibility for plugins using add_management_page().
	 * See wp-admin/admin.php for redirect from edit.php to tools.php.
	 */
	if ( 'tools.php' === $parent_slug ) {
		$_registered_pages[ get_plugin_page_hookname( $menu_slug, 'edit.php' ) ] = true;
	}

	// No parent as top level.
	$_parent_pages[ $menu_slug ] = $parent_slug;

	return $hookname;
}