[ CAKEPHP ]Question in generate Xml file within controller snippet

I need to generate xml file from controller after I find in google and found solution but I have question in snippet

1.how to access file in webroot for example in snippet

$xmlFile = fopen('/webroot/xml/xml.开发者_如何学编程xml', 'a');

thank you for advice

$xmlFile = fopen(WWW_ROOT . 'xml/xml.xml', 'a');