What do you mean by 'custom table'?

By custom table, l mean create a new database table.

In most cases, there's no need to create new tables in the database.

According to Elgg documentation, Elgg supports custom SQL queries using the database API.

Also, you can learn DBAL documentation.

@Hermand Pessek. If you strongly wanted to create your own table in addition to Elgg tables, you can do it by following below code block in the init( ) of the Bootstrap class. 

<?php

namespace your_plugin_name_or_any_preferred_name;

class Bootstrap extends DefaultPluginBootstrap {

   public function init( ) {
      $db = elgg( )->db;
      $prefix = $db->prefix;

      $query = "CREATE TABLE IF NOT EXISTS `{$dbprefix}your_table_name` (
         `id` int(11) unsigned NOT NULL auto_increment,
         `your_column_name` varchar(40) NOT NULL,
      ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 AUTO_INCREMENT=1 COLLATE=utf8mb4_general_ci;";
    
     $db->getConnection('write')->executeQuery($query);
   }
}

@seri_ng

It would be even better to do this in the activate() function of the Bootstrap. That way it only gets called when you activate the plugin and not on every request.

PS: in 12 years of developing for Elgg I never needed to create my own table, just saying ;)

@Jerome Bakker

That is indeed much better to put the script in the activate( ) :)