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The XML format for defining a post-install script in package.xml is documented here. This 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.