roundcubemail/plugins/example_addressbook/example_addressbook.php

<?php

require_once __DIR__ . '/example_addressbook_backend.php';

/**
 * Sample plugin to add a new address book
 * with just a static list of contacts
 *
 * @license GNU GPLv3+
 * @author Thomas Bruederli
 */
class example_addressbook extends rcube_plugin
{
    private $abook_id = 'static';
    private $abook_name = 'Static List';

    public function init()
    {
        $this->add_hook('addressbooks_list', [$this, 'address_sources']);
        $this->add_hook('addressbook_get', [$this, 'get_address_book']);

        // use this address book for autocompletion queries
        // (maybe this should be configurable by the user?)
        $config  = rcmail::get_instance()->config;
        $sources = (array) $config->get('autocomplete_addressbooks', ['sql']);
        if (!in_array($this->abook_id, $sources)) {
            $sources[] = $this->abook_id;
            $config->set('autocomplete_addressbooks', $sources);
        }
    }

    public function address_sources($p)
    {
        $abook = new example_addressbook_backend($this->abook_name);

        $p['sources'][$this->abook_id] = [
            'id'       => $this->abook_id,
            'name'     => $this->abook_name,
            'readonly' => $abook->readonly,
            'groups'   => $abook->groups,
        ];

        return $p;
    }

    public function get_address_book($p)
    {
        if ($p['id'] === $this->abook_id) {
            $p['instance'] = new example_addressbook_backend($this->abook_name);
        }

        return $p;
    }
}