Templates

GNU social uses the Twig template engine. When you handle a UI-related event, you add your own twig snippets either with App\Util\Formatting::twigRenderFile or App\Util\Formatting::twigRenderString.

Example

public function onAppendRightPanelBlock(array $vars, array &$res): bool
{
    if ($vars['path'] == 'attachment_show') {
        $related_notes = DB::dql('select n from attachment_to_note an ' .
    'join note n with n.id = an.note_id ' .
    'where an.attachment_id = :attachment_id', ['attachment_id' => $vars['vars']['attachment_id']]);
        $related_tags = DB::dql('select distinct t.tag ' .
    'from attachment_to_note an join note_tag t with an.note_id = t.note_id ' .
    'where an.attachment_id = :attachment_id', ['attachment_id' => $vars['vars']['attachment_id']]);
        $res[] = Formatting::twigRenderFile('attachmentShowRelated/attachmentRelatedNotes.html.twig', ['related_notes' => $related_notes]);
        $res[] = Formatting::twigRenderFile('attachmentShowRelated/attachmentRelatedTags.html.twig', ['related_tags' => $related_tags]);
    }
    return Event::next;
}

Regarding using the Twig language, you can refer to Twig Documentation.