Reply To: WordPress – modify title tag.

[Ilan Firsov]

I think you may be able to use the same logic in the filter you just need to return the new title at the end instead of echoing it
Try this

<?php
add_filter( 'pre_get_document_title', 'my_custom_page_title' );
function my_custom_page_title( $title ) {
    global $page, $paged;

    // Add the site name and separator.
    $new_title = get_bloginfo('name') . ' | ';

    // Add the site description for the front page.
    $site_description = get_bloginfo('description', 'display');
    if ($site_description && (is_front_page())) $new_title .= $site_description;

    // Show page title on posts page.
    if (is_home()) $new_title .= $title;

    // Title for 404.
    if (is_404()) $new_title .= 'file not available';

    // Add a page number if necessary:
    if ($paged >= 2 || $page >= 2 ) $new_title .= ' ' . sprintf( __('(page %s)'), max($paged, $page) );

    // Show page title.
    if (is_page()) $new_title .= $title;


    return $new_title;
}