src/Zend/Filter/Callback.php
public function filter($value)
    {
        $options = array();

        if ($this->_options !== null) {
            if (!is_array($this->_options)) {
                $options = array($this->_options);
            } else {
                $options = $this->_options;
            }
        }

        array_unshift($options, $value);

        return call_user_func_array($this->_callback, $options);
    }
public function filter($value)
    {
        $options = array();

        if ($this->_options !== null) {
            if (!is_array($this->_options)) {
                $options = array($this->_options);
            } else {
                $options = $this->_options;
            }
        }

        array_unshift($options, $value);

        return call_user_func_array($this->_callback, $options);
    }
Calls the filter per callback

@param mixed $value Options for the set callback
@return mixed       Result from the filter which was callbacked