Adding elements to an existing XML file

I have an existing xml file as such:

<comments>
  <comment page="string" author="string"
date="dateTime">string</comment>
  <comment page="string" author="string"
date="dateTime">string</comment>
</comments>

I want to be able to add more <comment> elements to this file from a web
form using PHP. It is possible that more than one user will attempt to
add an element at the same time. I am currently using Sablotron for XSL
transformations.

Eventually I wish to also add elements to another file in an ordered
fashion, but in this case simply appending a new <comment> element to
the end of the file (but inside the end </comments> tag of course) is
fine.

How would I go about doing this?

Thanks.

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