To reflect on a namespaced class in PHP 5.3, you must always specify the fully qualified name of the class - even if you've aliased the containing namespace using a "use" statement.
So instead of:
<?php
use App\Core as Core;
$oReflectionClass = new ReflectionClass('Core\Singleton');
?>
You would type:
<?php
use App\Core as Core;
$oReflectionClass = new ReflectionClass('App\Core\Singleton');
?>