The most popular PHP CMS might be Wordpress or Joomla but I use Drupal. Version 6, 7 and 8 have reached their end-of-life. I fell in love with their taxonomy core module which I think is the best and have been using it since. Starting from Drupal 9 (Current ver. is 10), it can also be deployed as content-as-service instead of just a front-end CMS. IMHO has one of the best documentation.
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