You need to upload the edited XML to server using HTTPService and save to file on the server using PHP, Python, etc.
You may have gotten the XML from the server using HTTPService and then stuffed it in an object in Flex, but after editing the XML you need to use HTTPService to send it back to the server for writing to file.
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Using the Flex Application?
Have you seen this done before
Edit the xml file in flex, have the flex application overwrite file on server.
yes, I have done it. Part of the app I am working on takes an XML "report" format to display data. The user can reconfigure parts of this and then optionally save the results. The XML file is shipped back via HTTPService and an ASP or JSP page does the actual save. At present because the file size is small I am just using querystring parameters but anticipate this will have to change in time as report complexity and hence size increases. Hence I do some encoding of the file to ensure that all characters are legal ones for this transfer, and then decode at the server end.
In my case the report editing is not done by the user manipulating the XML directly, but by GUI interface choices such as moving things around on screen.