<?php
class Usuarios {

    function listar() {
        require_once 'controller/conexao.php';

        $sql = "SELECT * FROM usuarios";
        $usuarios = $mysqli->query($sql);

        while ($linhas = $usuarios->fetch_array()) {
            echo '<p>'.$linhas['id'].'</p>';
        }
    }

    function adicionar() {
        require_once 'controller/conexao.php';
    }
}

<?php

require_once('model/Usuarios.class.php');

$user = new Usuarios();
$user->listar();

//$user->adicionar();

<?php

define('SITE_URL', 'http://localhost/sistemas/objetos');

$host = 'localhost';
$user = 'root';
$pass = '1';
$db   = 'wellisson';

$mysqli = new mysqli($host, $user, $pass, $db);
$mysqli->set_charset('utf8');

if ($mysqli->connect_error):
    echo "Erro na conexao: " . $mysqli->connect_errno . $mysqli->connect_error;
endif;