You can't explain that.

namespaces.php

<?php
namespace SDM\DB
{
	class MySQL
	{
		public function __construct ()
		{
			echo 'MySQL->__construct()', "\n";
		}
	}
}

namespace
{
	ini_set('display_errors', 1); 
	error_reporting(-1);
?>
<pre>
<?php
	use SDM\DB;
	$db = new MySQL;
}

Output:

Fatal error: Class 'MySQL' not found in /path/to/namespaces.php on line 21