Drop a file called "foaf_header.php" into your site's wp-content/plugins/ and fill it with the following text:
<?php
/*
Plugin Name: Foaf Header
Version: 0.5
Plugin URI: http://www.cozy.org/wordpress/foaf-header.html
Description: Insert link to an associated FOAF (Friend of a friend).
Author: Ben Hyde
Author URI: http://enthusiasm.cozy.org/
*/
function insert_foaf_meta_tag() {
echo <<<EOT
<link rel="meta" href="http://example.com/my/foaf.rdf"
type="application/rdf+xml" title="FOAF" />
EOT;
}
add_action('wp_head', 'insert_foaf_meta_tag',3);
?>
Be sure to modify "http://example.com/my/foaf.rdf" to point to your actual foaf file. Save the file, activate the plug-in by going to your wordpress adminstration UI, visiting the plug-in page and hitting activate.
That's it.
You should see the added link on the source of your pages, near the bottom of the <head> section.
Meanwhile FOAF Output Plugin is much more full featured, with some really cool output.