PHP not Updating for Anonymous Users
Looking at the code of the PHP filter:

class Php extends FilterBase {

  /**
   * {@inheritdoc}
   */
  public function process($text, $langcode) {
    $result = new FilterProcessResult(php_eval($text));
    $result-&gt;setCacheMaxAge(0);
    return $result;
  }


https://git.drupalcode.org/project/php/blob/8.x-1.x/src/Plugin/Filter/Php.php

it sets already a cache max age of 0, so you only need to remove the page cache for anonymous users by uninstalling the module "Internal Page Cache".