如何在自动加载中使用PHP命名空间？

project/
    src/
        Namespace1/
            Class1.php
            Class2.php
        Namespace2/
            Class1.php
            Class2.php

<?php
namespace Namespace1;

class Class1
{
    // ...
}

<?php
function autoload($className)
{
    $className = ltrim($className, '\\');
    $filePath = __DIR__ . '/src/' . str_replace('\\', '/', $className) . '.php';

    if (file_exists($filePath)) {
        require $filePath;
    } else {
        echo "The file $filePath does not exist.";
    }
}

spl_autoload_register('autoload');

<?php
use Namespace1\Class1;
use Namespace2\Class1 as Class1FromNamespace2;

$class1 = new Class1();
$class1FromNamespace2 = new Class1FromNamespace2();