How to properly pass and return by reference between function inside a class

Question

I create a class that hold mysqli object link with a private function to generate params that needed by bind_params function which then will be used by this function:

call_user_func_array(array($stmt, 'bind_param'), $params);

Since, bind_param need to pass value by reference, so I create a the function like this:

// $values is taken from $_POST's values
private function &params($values) {

    // Generate first value for bind_params like: 'ssis'
    $param_type = '';
    foreach ($values as $value) {
        $param_type .= gettype($value)[0];
    }

    // Generate the rest value for bind_params
    $params[] = & $param_type;
    for ($i = 0; $i < count($values); $i++) {
        $params[] = & $values[$i];
    }

    // The returned values should be like { 'ssi', $str_value, $str_value, $int_value }
    return $params;
}

How I execute prepared statement is like this:

function insert_prepared($query, $posted) {
    $params = &$this->params(array_values($posted));
    $stmt = $this->mysqli->prepare($query);
    if ($stmt) {
        call_user_func_array(array($stmt, 'bind_param'), $params);
        if ($stmt->execute()) {
            return true;
        } else {
            return false;
        }
    }
}

Okay, now the problem is why call_user_func_array(array($stmt, 'bind_param'), $params) still complaints about expected to be a reference?

Did the manual said the function should be written like function &params($values)?

What did I missing?