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Either way, you should not add this to the footer.php.
Footer.php does not add the footer, but instead it post-processes the content to be shown and invokes the display of the theme.
If you want something to be shown in the footer of every page either edit your theme(s) to include the code or add it to the footer field in system admin -> preferences -> Meta Tags and Footer.
Not all themes use the XOOPS footer, though.