How to get the 'return type' of a function?

Hello guys, I try to use 'ReflectionFunction' to retrieve the info of a  
function.
For example:
<?php

$rf = new ReflectionFunction('strstr');
echo $rf;
?>
=============== output ==================

Function [ <internal:standard> function strstr ] {

   - Parameters [3] {
     Parameter #0 [ <required> $haystack ]
     Parameter #1 [ <required> $needle ]
     Parameter #2 [ <optional> $part ]
   }
}

The problem is there's no 'return type' (i.e. 'string' in this example)
info about the function.

Could you tell me how to retrieve the 'return type'?
Thanks.


-- 
Dasn