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In PHP5, register_long_arrays must be on in order for the old-style global arrays $HTTP_GET_VARS, $HTTP_POST_VARS, etc. to be recognized.
I don't think the current XOOPS core uses these ($HTTP_*_VARS), but I'm not sure. Even if the current XOOPS core does use these variables, I would expect that dependency to be removed in the near future.
But there are definitely third-party modules that still use the $HTTP_*_VARS arrays.
According to the PHP manual, register_long_arrays can be turned on (or off) in an .htaccess file, so that provides a workaround, provided that your server runs Apache and that Apache is configured to allow you to override PHP configuration values in an .htaccess file.