src/Zend/Filter.php
public function addFilter(Zend_Filter_Interface $filter, $placement = self::CHAIN_APPEND)
    {
        if ($placement == self::CHAIN_PREPEND) {
            array_unshift($this->_filters, $filter);
        } else {
            $this->_filters[] = $filter;
        }
        return $this;
    }
public function addFilter(Zend_Filter_Interface $filter, $placement = self::CHAIN_APPEND)
    {
        if ($placement == self::CHAIN_PREPEND) {
            array_unshift($this->_filters, $filter);
        } else {
            $this->_filters[] = $filter;
        }
        return $this;
    }
Adds a filter to the chain

@param  Zend_Filter_Interface $filter
@param  string $placement
@return Zend_Filter Provides a fluent interface