How do I use PHP namespaces with autoload?

As mentioned Pascal MARTIN, you should replace the '\' with DIRECTORY_SEPARATOR for example:

$filename = BASE_PATH . DIRECTORY_SEPARATOR . str_replace('\\', DIRECTORY_SEPARATOR, $class) . '.php';
include($filename);

Also I would suggest you to reorganize the dirrectory structure, to make the code more readable. This could be an alternative:

Directory structure:

ProjectRoot
 |- lib

File: /ProjectRoot/lib/Person/Barnes/David/Class1.php

<?php
namespace Person\Barnes\David
class Class1
{
    public function __construct()
    {
        echo __CLASS__;
    }
}
?>

• Make the sub directory for each namespace you are defined.

File: /ProjectRoot/test.php

define('BASE_PATH', realpath(dirname(__FILE__)));
function my_autoloader($class)
{
    $filename = BASE_PATH . '/lib/' . str_replace('\\', '/', $class) . '.php';
    include($filename);
}
spl_autoload_register('my_autoloader');

use Person\Barnes\David as MyPerson;
$class = new MyPerson\Class1();

• I used php 5 recomendation for autoloader declaration. If you are still with PHP 4, replace it with the old syntax: function __autoload($class)

Class1 is not in the global scope.

Note that this is an old answer and things have changed since the days where you couldn't assume the support for spl_autoload_register() which was introduced in PHP 5.1 (now many years ago!).

These days, you would likely be using Composer. Under the hood, this would be something along the lines of this snippet to enable class autoloading.

spl_autoload_register(function ($class) {
    // Adapt this depending on your directory structure
    $parts = explode('\\', $class);
    include end($parts) . '.php';
});

For completeness, here is the old answer:

To load a class that is not defined in the global scope, you need to use an autoloader.

<?php

// Note that `__autoload()` is removed as of PHP 8 in favour of 
// `spl_autoload_register()`, see above
function __autoload($class)
{
    // Adapt this depending on your directory structure
    $parts = explode('\\', $class);
    require end($parts) . '.php';
}

use Person\Barnes\David as MyPerson;

$class = new MyPerson\Class1();

or without aliases:

use Person\Barnes\David\Class1;

$class = new Class1();