# Callback

This filter allows you to use own methods in conjunction with `Zend_Filter`. You don't have to create a new filter when you already have a method which does the job.

Let's expect we want to create a filter which reverses a string.

    $filter = new Zend_Filter_Callback('strrev');

    print $filter->filter('Hello!');
    // returns "!olleH"

As you can see it's really simple to use a callback to define a own filter. It is also possible to use a method, which is defined within a class, by giving an array as callback.

    // Our classdefinition
    class MyClass
    {
        public function Reverse($param);
    }

    // The filter definition
    $filter = new Zend_Filter_Callback(array('MyClass', 'Reverse'));
    print $filter->filter('Hello!');

To get the actual set callback use `getCallback()` and to set another callback use `setCallback()`.

It is also possible to define default parameters, which are given to the called method as array when the filter is executed. This array will be concatenated with the value which will be filtered.

    $filter = new Zend_Filter_Callback(
        array(
            'callback' => 'MyMethod',
            'options'  => array('key' => 'param1', 'key2' => 'param2')
        )
    );
    $filter->filter(array('value' => 'Hello'));

When you would call the above method definition manually it would look like this:

    $value = MyMethod('Hello', 'param1', 'param2');

You should note that defining a callback method which can not be called will raise an exception.