tag.php

<?php

// https://getkirby.com/docs/cookbook/tags

return function($site, $pages, $page) {

  // fetch the basic set of pages
  $projects = page('projects')->children()->visible()->flip();
  // fetch the basic set of posts
  $blogposts = page('blog')->children()->visible()->flip();

  // add the tag filter
  if($tag = param('tag')) {
    $projects = $projects->filterBy('tags', $tag, ',');
    $blogposts = $blogposts->filterBy('tags', $tag, ',');
  }

  // fetch all tags
  $tags =  $projects->pluck('tags', ',', false);
  $tags += $blogposts->pluck('tags', ',', false);

  // apply pagination
  $projects   = $projects->paginate(10);
  $pagination = $projects->pagination();
  $blogposts  = $blogposts->paginate(10);
  $pagination = $blogposts->pagination();

  return compact('projects', 'blogposts', 'tags', 'tag', 'pagination');

};