Misanderstanding how to proper use callback while filtering in php

I want to properly check the user data from a form in PHP. I'm using filter class. I need to use a callback function to check the password. To achieve this, I used this tutorial on twentieths point (callback filter): http://www.phpro.org/tutorials/Filtering-Data-with-PHP.html#21

However I still get this error:

"Warning: filter_var(): First argument is expected to be a valid callback" error.

Here is my class :

class Callback {

private function validPasswd($sPasswd) {

        $r1 = '/#[A-Z]+#/';  //at least one uppercase
        $r2 = '/#[a-z]+#/';  //at least one lowercase
        $r3 = '/#[!@#$%^&*()\-_=+{};:,<.>]+#/';  // at least one special char
        $r4 = '/#[0-9]+#/';  //at least one number

        if (preg_match($r1, $sPasswd)) {
            return false;
        }
        if (preg_match_all($r2, $sPasswd)) {
            return false;
        }
        if (preg_match_all($r3, $sPasswd)) {
            return false;
        }
        if (preg_match_all($r4, $sPasswd)) {
            return false;
        }
        if (strlen($sPasswd) < 8) {
            return false;
        }
        return $sPasswd;
    }
}

and here's the part of my method:

    public function createAccountAction() {
    //get password from form to verification.
    $sPasswd = $_POST['password'];

    //prepare all values to be checked with a filter
    $aOptions = array(
        'first-name' => FILTER_SANITIZE_STRING,
        'last-name' => FILTER_SANITIZE_STRING,
        'login' => FILTER_SANITIZE_STRING,
        'password' => array(
            'filter' => FILTER_CALLBACK,
            'options' => 'validPasswd'
        ),
        'email' => FILTER_SANITIZE_EMAIL,
        'conf-email' => FILTER_SANITIZE_EMAIL
    );
    var_dump(filter_var($sPasswd, FILTER_CALLBACK, array("options" => array("Callback"=>"validPasswd"))));

namespace and use are correct. I think it's the way I call the method that is not good.