How are you going to read individual files in PHP? I know that you can use QueryPath, but this seems a little redundant, I just want to read the file.
odt, zip files are compressed xml.
If all you have to do is read the raw file. Just unzip it and read it as a regular file.
If you need to parse useful text, then you need QueryPath or other xslt syntax.
OpenTBS can read and modify OpenDocument files in PHP.
Since OpenDocument files are XML files stored in a zip archive, you can also use the TbsZip class to easily read a zip archive under PHP without any other library dependency.
http://pear.php.net/package/OpenDocument may be what you need. However, I did not use it myself.