StringToTitle filter

I like the filtering capabilities of the Zend Framework, but for some reason there doesn’t seem to be a string to title case filter (though there is a string to upper and string to lower). So here it is:

< ?php

/**
 * @see Zend_Filter_Interface
 */
require_once 'Zend/Filter/Interface.php';

/**
 * Convert a string to titlecase.
 *
 * Seemingly missing from the core Zend distribution.
 */
class StringToTitle implements Zend_Filter_Interface
{
    /**
     * Encoding for the input string
     *
     * @var string
     */
    protected $_encoding = null;

    /**
     * Set the input encoding for the given string
     *
     * @param  string $encoding
     * @throws Zend_Filter_Exception
     */
    public function setEncoding($encoding = null)
    {
        if (!function_exists('mb_convert_case')) {
            require_once 'Zend/Filter/Exception.php';
            throw new Zend_Filter_Exception('mbstring is required for this feature');
        }
        $this->_encoding = $encoding;
    }

    /**
     * Defined by Zend_Filter_Interface
     *
     * Returns the string $value, converting characters to titlecase as necessary
     *
     * @param  string $value
     * @return string
     */
    public function filter($value)
    {
        if ($this->_encoding) {
            return mb_convert_case((string) $value, MB_CASE_TITLE, $this->_encoding);
        }
        return ucwords((string) $value);
    }
}