Example structure for http://stackoverflow.com/q/10542012

bootstrap.php

<?php

// file: ./bootstrap.php

spl_autoload_register(function($className) {
    $className = ltrim($className, '\\');
    $fileName  = '';
    $namespace = '';
    if ($lastNsPos = strripos($className, '\\')) {
        $namespace = substr($className, 0, $lastNsPos);
        $className = substr($className, $lastNsPos + 1);
        $fileName  = str_replace('\\', DIRECTORY_SEPARATOR, $namespace) . DIRECTORY_SEPARATOR;
    }
    $fileName .= str_replace('_', DIRECTORY_SEPARATOR, $className) . '.php';
    var_dump($fileName); // to see which file is loaded
    require $fileName;
});

new Shape\Circle();

Circle.php

<?php

// file: ./Shape/Circle.php

namespace Shape;

class Circle extends Shape implements ShapeInterface {
    public function __construct() {
        var_dump($this);
    }
}

Shape.php

<?php

// file: ./Shape/Shape.php

namespace Shape;

abstract class Shape {}

ShapeInterface.php

<?php

// file: ./Shape/ShapeInterface.php

namespace Shape;

interface ShapeInterface {}