Самая популярная либа для PHP PEAR.

Вот код функции преобразования простой строки в XML строку.

    /**
    * Escape XML entities.
    *
    * @param string xml
    * @return string xml
    * @access private
    */
    function _xml_entities($xml) {
        $xml = str_replace(array('ь', 'Ь', 'ц',
                                 'Ц', 'д', 'Д',
                                 'Я'
                                ),
                         array('ü', 'Ü', 'ö',
                                 'Ö', 'ä', 'Ä',
                                 'ß'
                                ),
                         $xml
                         );

        $xml = preg_replace(array("/\&([a-z\d\#]+)\;/i",
                                 "/\&/",
                                 "/\#\|\|([a-z\d\#]+)\|\|\#/i",
                                 "/([^a-zA-Z\d\s\<\>\&\;\.\:\=\"\-\/\%\?\!\'\(\)\[\]\{\}\$\#\+\,\@_])/e"
                                 ),
                            array("#||\\1||#",
                                 "&amp;",
                                 "&\\1;",
                                 "'&#'.ord('\\1').';'"
                                 ),
                            $xml
                         );

        return $xml;
    }