Class Not found when autoloading with namespace (no Composer) - php

Question

I have been checking for answers but with no success. I'm not using composer autoload function.
I'm trying to test some classes, Collections and traits in a index.php.
The problem is when i'm including a namespace, I'have created the _autoload function (No composer) to load a class when needed. If I execute php index.php i'm getting the class not found error.
I have tried the use command, but with no success as well.
It solves when I include each class one by one like this:
include_once __DIR__ . '/Collection.php';
include_once __DIR__ . '/User.php';
include_once __DIR__ . '/Tweet.php';
include_once __DIR__ . '/UserCollection.php';
include_once __DIR__ . '/TweetCollection.php';
Thats my index.php:
<?php
namespace phpexercises\entregableT3;
error_reporting(E_ALL);
ini_set( "display_errors", "on" );
/*
include_once __DIR__ . '/Collection.php';
include_once __DIR__ . '/User.php';
include_once __DIR__ . '/Tweet.php';
include_once __DIR__ . '/UserCollection.php';
include_once __DIR__ . '/TweetCollection.php';
*/
use phpexercises\entregableT3\User;
function __autoload($classname){
require __DIR__ . "/" . $classname . ".php";
}
//UserCollection pruebas
$alex = new User();
$jumbo = new User();
$users = new UserCollection();
$alex->phone = "682383";
$alex->email = "alex#example.com";
$alex->city = "BCN";
$alex->gender = "Male";
$jumbo->phone = "54534535";
$jumbo->email = "jumbo#example.com";
$jumbo->city = "elche";
$jumbo->gender = "Male";
$users->add($alex);
$users->add($jumbo);
print_r($users->findByEmail("alex#example.com"));
print_r($users->findByGender("Female"));
//Tweet Collection pruebas
$tweet = new Tweet();
$tweet->email = "alex#example.com";
$tweet->text = "This is a tweet";
$tweet->date = date("D:M:Y");
$tweet1 = new Tweet();
$tweet1->email = "alex#example.com";
$tweet1->text = "This is a tweet";
$tweet1->date = date("D:M:Y");
When I uncomment the requires, everything works fine, but it is not the way to load all classes...Any idea for this behaviour? How should I load the classes at same time?
Thanks