autoload.php

<?php
$whereAmI = dirname(realpath(__FILE__)). DIRECTORY_SEPARATOR;

set_include_path(
    implode(
        PATH_SEPARATOR,
        array_unique(
            array_merge(
                array(
                    $whereAmI . 'src',
                    $whereAmI . 'tests',
                ),
                explode(PATH_SEPARATOR, get_include_path())
            )
        )
    )
);

spl_autoload_register(
    function ($qname)
    {
        $qname2path = stream_resolve_include_path(
            str_replace('\\', DIRECTORY_SEPARATOR, $qname) . '.php');

        if ($qname2path !== false) {
            require $qname2path;
        }
    }
);

index.php

<?php

include 'autoload.php';

use classe\Usuario;

$u = new Usuario;
var_dump($u);

Usuario.php

<?php

// dir classe/Usuario

namespace classe;

class Usuario
{
    private $email;
    private $senha;
	
    public function setEmail($email)
    {
       $email = strtolower($email);
       
       if(!filter_var($email, FILTER_VALIDATE_EMAIL))
        return false;
       else	
         $this->email = $email;
    }
    public function getEmail()
    {
    	return $this->email;
    }

 }