Class Method with call_user_func

Can anyone tell me how this can be done?
I'm trying to invoke a private function in my class with another private function using the call_user_func function:

call_user_func($this->$replacement_func,$replacement_params);

The call_user_func function should (theoretically) call the private function, but instead is throwing this error:

First argument is expected to be a valid callback

Which obciously means that this is the wrong way of doing this. Does anybody know the proper way of going about this?

EDIT:

Here's how you would do this:
call_user_func(array("classname",$replacement_func),$replacement_params);
Where classname is the name of your class.

Hello,
You need to pass the instance in by an array too like the following:


Here's an example where I've used it:

/**
* DetectRequestType - Checks to see all the keys and values of the post request
* as the post can contain more than one request, look through the ones we need for
* returned data.
*
* $param $postReq - Reference of the $_POST superglobal to search.
*/
public function DetectRequestType(&$postReq)
{
foreach($postReq as $key => $post)
{
# Find first and only valid request to return data on ^^
if(in_array($key, (array)$this->_postTypes))
{
$methodName = (string)"Get" . $key . "Data";

if(method_exists($this, $methodName))
{
return (string)(call_user_func(array($this, $methodName), intval($post)));
}
}
}
}