SimpleXML: convert xml to text

Hello, I want to convert some xml stuff to simple string using php, say:

<?php

$string = <<<XML
<a> left <foo>center1<ok>ok</ok>center2</foo>right </a>
XML;

$xml = simplexml_load_string($string);
echo $xml;
?>
The code will output "left right" while all the central stuff was lost.
How should I print it as "left center1 ok center2 right" ?

Thanks in advance, and please Cc me. :)

-- 
Dasn