Shows how to send a Participants Database private link by matching 2 or more fields

pdb-custom-link-recovery.php

<?php
/**
 * Plugin Name: PDB Custom Link Recovery
 * Description: sets up a private link recovery using two or more fields to find the record
 */
add_filter( 'pdb-find_record_match', 'xnau_custom_link_recovery', 10, 3 );

/**
 * finds a match for a link recovery submission
 * 
 * @global wpdb $wpdb
 * 
 * @param int|bool  $matched_id the id found using the standard method, bool 
 *                              false if no match was found
 * @param string    $columns column name or names used to find the match
 * @param array     $submission the un-sanitized $_POST array
 * 
 * @return int|bool the found record ID
 */
function xnau_custom_link_recovery ( $matched_id, $columns, $submission )
{
  // test to make sure it is the correct form
  if ( $submission['action'] !== 'retrieve' ) {
    return $matched_id;
  }
  
  global $wpdb;
  
  // this array names the fields to test for a match
  $match_fields = array( 'email', 'first_name' );
  
  $matches = array();
  $values = array();
  foreach( $match_fields as $field ) {
    
    // add the match clause to the query
    $matches[] = '`' . $field . '` = %s';
    
    // add the value to the values array
    $values[] = isset( $submission[$field] ) ? $submission[$field] : '';
  }
  
  // set up the query
  $sql = 'SELECT `id` FROM ' . Participants_Db::$participants_table . ' WHERE ' . implode( ' AND ', $matches );
  
  // get the results of the match
  $result = $wpdb->get_var( $wpdb->prepare( $sql, $values ) );
  
  // return the result if it is an ID, false otherwise
  return is_numeric( $result ) ? $result : false;
}

Installing a plugin from a GitHub Gist:

https://xnau.com/how-to-install-a-wordpress-plugin-from-a-gist/