Snippet to rewrite the "no access" message

pmpro-custom-content-message.php

<?php
/*
 * Rebuilds the "no access" message before Paid Memberships Pro applies any substitution.
 * Parses current post for [membership] shortcodes as needed.
 * Here I'm just getting rid of the "Oxford comma", but it can be used for
 * rebuilding the message any way you like.
 */

//the filters are called by PMPro just before any "!!" variable substitution:
// https://github.com/strangerstudios/paid-memberships-pro/blob/2190e731fbb7ccebfa07f246f926859de014e79a/includes/functions.php#L1928
add_filter('pmpro_non_member_text_filter', 'pp_pmpro_custom_content_message', 10, 2);
add_filter('pmpro_not_logged_in_text_filter', 'pp_pmpro_custom_content_message', 10, 2);
//if you also need to filter rss content...
//add_filter('pmpro_rss_text_filter', 'pp_pmpro_custom_content_message', 10, 2); 

function pp_pmpro_custom_content_message($text) {
  //get current post
  $post = get_post();
  $levels = array();
  
  //if post has a [membership] shortcode, parse it to find out which levels are needed to acces content
  if(has_shortcode($post->post_content, 'membership')) {
    preg_match_all( 
      '/' . get_shortcode_regex(array('membership')) . '/', 
      $post->post_content, 
      $matches, 
      PREG_SET_ORDER
    );
    
    foreach($matches as $match) {
      $level_ids = array();
      
      //check if shortcode actually shows the "noaccess" message. 
      if(strpos($match[0], 'show_noaccess="true"') >=0) {
        preg_match('/level="([0-9,-]+)"/', $match[3], $level_ids);

        $level_ids = explode(',', $level_ids[1]);
        
        //get all the level names needed
        $levels = array_map(function($id) {
          if(strpos($id, '-') === false) {
            return pmpro_getLevel($id)->name;
          }
        } , $level_ids);

        //don't check any further (I'm assuming there aren't two shortcodes with "noacces" 
        //for different levels in the same post).
        break;
      }
    }
  }
  
  //if the post doesn't contain a "noaccess" shortcode, get levels set on the post itself
  if(empty($levels)) {
    $levels = pmpro_has_membership_access($post->ID, NULL, true);
    $levels = $levels[2];
  }

  //break the last level to apply a conjunction before it
  $last = array_pop($levels);

  //build the string
  if(count($levels) > 0) {
    //                     here's the conjunction ↓
    $levels_string = (implode(', ', $levels) . ' and ' . $last);
  } else {
    //if there's only one level...
    $levels_string = $last;
  }

  //replace the !! variables for PMPro. If you leave any variables here, PMPro will replace them normally
  $text = str_replace('!!levels!!', $levels_string, $text);
  
  return $text;
}