Example showing proposed dynamic referencing of "Page" class namespace in SilverStripe 4

Foo\Page.php

<?php

namespace Foo;

class Page {
  function whoami() {
    echo __CLASS__;
  }
}

test.php

<?php

// This is the equivalent of `$project`, but for the namespace Page lives in
$appNamespace = "Foo";

// This is a fake PSR4 autoloader - normally composer would provide
spl_autoload_register(function ($class_name) {
  $filename = str_replace('\\', '/', $class_name) . '.php';
  if (file_exists($filename)) {
    include $filename;
    return true;
  }
  return false;
});

// This would normally be in Core.php or something like that.
spl_autoload_register(function ($class_name) {
  global $appNamespace;

  if ($class_name == '__App\Page') {
    class_alias("$appNamespace\Page", '__App\Page');
    return true;
  }
  return false;
});

// Here we reference _App\Page, and the autoload dynamically creates the
// alias to Foo\Page
class BlogPage extends __App\Page {
}

// We then create one and call the base class' whoami method to show
// that __CLASS__ isn't affected by the class_alias
$b = new BlogPage();
$b->whoami();