Zend_Filter supplies a set of commonly needed filters, but developers will often need to write custom filters for their particular use cases. The task of writing a custom filter is facilitated by implementing Zend_Filter_Interface.

Zend_Filter_Interface defines a single method, filter(), that may be implemented by user classes. An object that implements this interface may be added to a filter chain with Zend_Filter::addFilter().

The following example demonstrates how to write a custom filter:

class MyFilter implements Zend_Filter_Interface
{
    public function filter($value)
    {
        // perform some transformation upon $value to arrive on $valueFiltered

        return $valueFiltered;
    }
}

To add an instance of the filter defined above to a filter chain:

$filterChain = new Zend_Filter();
$filterChain->addFilter(new MyFilter());