How does ReflectionFunction's isDeprecated work?

How does ReflectionFunction's isDeprecated work?

I'm trying to use the following function to loop through loaded files and
find depreciated functions.
//loads all files
include dirname(__FILE__) . '/loader.php';
$functions = get_defined_functions();
foreach ($functions['user'] as $func) {
$rf = new ReflectionFunction('$func');
var_dump($rf->isDeprecated());
}
Some functions have markup like the following, yet it's still return
false. In fact every single function returns false yet there are a lot
with markup stating @deprecated.
**
*
* @since 0.71
* @deprecated 1.5.1
* @deprecated Use get_post()
*
* @param int $postid
* @return array
*/
ref: http://www.php.net/manual/en/class.reflectionfunction.php