Shows how to limit the number of Participants Database signups

pdb-limit-signups.php

<?php
/**
 * Plugin Name: PDB Limit Signups
 * Description: Adds a count of the number of Participants Database signups, and prevents new signups 
 *              once a number of registrations has been reached
 */
add_filter( 'pdb-signup_shortcode_output', 'xnau_limit_signups' );
/**
 * adds a display of the number of signups and prevents signups once a limit is reached
 *
 * @param string $output the shourtcode output HTML
 * @return string the output HTML
 */
function xnau_limit_signups( $output )
{
  // set the signup limit here
  $signup_limit = 100;
  
  /*
   * get the current count
   *
   * note: you can add a "filter" attribute to this shortcode if you only want to count a certain kind 
   * of record: see https://xnau.com/list-shortcode-filters/
   */
  $signup_count = do_shortcode('[pdb_total]');
  
  // this is added to the display to show the count
  $count_display = '<h3>Current Count: ' . $signup_count . ' Registrations</h3>';
  
  // this is shown once the limit is reached
  $overlimit_message = '<h3>' . $signup_count . ' Registrations Have Been Submitted, Registration is Now Closed</h3>';
  
  if ( $signup_count < $signup_limit ) {
    // still under the limit, show the count and the signup form
    return $count_display . $output;
  } else {
    // we've hit the limit, show the message without the form
    return $overlimit_message;
  }
}