Saving XML to a specific location

I have created an XML file using PHP. I am using the DOMDocument method save() to save the DOM to a file. My question is: is it possible to specify where I want the file to be placed? For instance if I have the script in a subdirectory, could I then specify that I want the file saved in the root directory?

Also I take it I would have to have the correct permissions to write to folders?

Alternately is it possible to download the file once it is generated? I am using PHP.