In Zend_Filter_StripTags, if the filter is applied only for strings, returns the original value if the parameter is not a string.

    /**
     * Defined by Zend_Filter_Interface
     *
     * @todo improve docblock descriptions
     *
     * @param  string $value
     * @return string
     */
    public function filter($value)
    {
        // If the param is not a string, return the original value
        if (!is_string($value)) {
            return $value;
        }
        
        $valueCopy = (string) $value;

        // If comments are allowed, then replace them with unique identifiers
        if ($this->commentsAllowed) {
            preg_match_all('/<\!--.*?--\s*>/s' , (string) $valueCopy, $matches);
            $comments = array_unique($matches[0]);
            foreach ($comments as $k => $v) {
                $valueCopy = str_replace($v, self::UNIQUE_ID_PREFIX . $k, $valueCopy);
            }
        }

        // Initialize accumulator for filtered data
        $dataFiltered = '';
        // Parse the input data iteratively as regular pre-tag text followed by a
        // tag; either may be empty strings
        preg_match_all('/([^<]*)(<?[^>]*>?)/', (string) $valueCopy, $matches);
        // Iterate over each set of matches
        foreach ($matches[1] as $index => $preTag) {
            // If the pre-tag text is non-empty, strip any ">" characters from it
            if (strlen($preTag)) {
                $preTag = str_replace('>', '', $preTag);
            }
            // If a tag exists in this match, then filter the tag
            $tag = $matches[2][$index];
            if (strlen($tag)) {
                $tagFiltered = $this->_filterTag($tag);
            } else {
                $tagFiltered = '';
            }
            // Add the filtered pre-tag text and filtered tag to the data buffer
            $dataFiltered .= $preTag . $tagFiltered;
        }

        // If comments are allowed, then replace the unique identifiers with the corresponding comments
        if ($this->commentsAllowed) {
            foreach ($comments as $k => $v) {
                $dataFiltered = str_replace(self::UNIQUE_ID_PREFIX . $k, $v, $dataFiltered);
            }
        }

        // Return the filtered data
        return $dataFiltered;
    }