<?php

// Connect to an external database
$db = Craft::createComponent([
    'emulatePrepare'    => true,
    'charset'           => 'utf8',
    'class'             => 'CraftDbConnection',
    'autoConnect'       => true,
]);

$db->connectionString = 'mysql:host=localhost;dbname=MY_DB;port=3306';
$db->username = 'UN';
$db->password = 'PW';

// Use Query Builder to fetch data from the database
$data = $db->createCommand()
    ->select("*")
    ->from("TBL")
    ->queryAll();

// Pass along the data to be used in a template via variables
return $data;