In XML, you can't assume that the doctype will include the same character entity definitions as HTML. XML authors may require character references instead. The following two functions use get_html_translation_table() to encode data in numeric references. The second, optional argument can be used to substitute a different translation table.
function xmlcharacters($string, $trans='') {
$trans=(is_array($trans))? $trans:get_html_translation_table(HTML_ENTITIES, ENT_QUOTES);
foreach ($trans as $k=>$v)
$trans[$k]= "&#".ord($k).";";
return strtr($string, $trans);
}
function xml_character_decode($string, $trans='') {
$trans=(is_array($trans))? $trans:get_html_translation_table(HTML_ENTITIES, ENT_QUOTES);
foreach ($trans as $k=>$v)
$trans[$k]= "&#".ord($k).";";
$trans=array_flip($trans);
return strtr($string, $trans);
}