The XML format for defining a post-install script in package.xml is documented
document describes the required elements for the PHP post-install script itself.
Post-install script files can be named anything one desires, but the class within
the file must be the same name as the file with all path separators replaced by
underscores. In other words, this postinstall script:
Must contain a class named Path_To_Script. Due to
casing differences between operating systems, it is recommended to always use
lowercased file names.