You can use the wp_title
filter to modify the HTML document title, and you can use the wp_head
action to hook in a custom HTML meta link. e.g.:
<?php
function wpse46249_filter_wp_title( $title ) {
// $title contains the default document title
// output by WordPress. Modify it here however
// you want.
$some_custom_title_content="CUSTOM TITLE CONTENT HERE";
// Now append it to the default
$custom_title = $title . $some_custom_title_content;
// Then return the result
return $custom_title;
}
add_filter( 'wp_title', 'wpse46249_filter_wp_title' );
function wpse46249_add_meta_link() {
echo '<meta name="keywords" content="INSERT KEYWORDS HERE" />';
}
add_action( 'wp_head', 'wpse46249_add_meta_link' );
?>
These callbacks can be added to a Plugin, to keep them safe from Theme switching.
For further reference, I would recommend taking a look at how the popular SEO Plugins implement these extensions.