Into the footer of what?
Using the content.tpl is not advised as it is smarty code.
What you need is to copy and paste this code into your php document where you want your links to display. This is commonly the config.inc.php file of your template.
The easiest way (as a hint on how to do in general) could be:
Code: Select all
ob_start();
include '/full/path/to/backlinkscode.php';
$bl_content = ob_get_contents();
ob_end_clean();
$serendipity['smarty']->assign('block_backlinks', $bl_content);
which then could be placed into your index.tpl file as {$block_backlinks}.
Or you may also could use the Plugin serendipity_event_externalphp.