Shows how to set up a duplicate record check on a link-type field

pdb-check-link-match.php

<?php
/**
 * Plugin Name: PDB Check Link Field for Duplicate
 * Description: make sure a new record does not match the link of an existing record
 * Version: 1.1
 */
 
 // this filter checks the submission
add_action( 'pdb-incoming_record_match_object', 'xnau_check_for_link_match' );
  
// set up our custom feedback message
add_filter( 'pdb-validation_error_messages', 'xnau_check_for_match_feedback' );

/**
 * checks the incoming submission for a match in the database
 *
 * @global wpdb $wpdb
 * @param \PDb_submission\incoming_record_match $record_match the record match object
 */
function xnau_check_for_link_match( $record_match )
{ 
  // set this to the name of the "link" field you want to check
  $check_field = 'link';
  
   // assume no match
  $match_status = false;
  
  // this is the submitted data
  $post = $record_match->post;
  
  /* 
   * don't include the record id to prevent attempting to add a new record with 
   * the same id as an existing record
   * 
   */
  if ( isset( $post['id'] ) ) {
    unset( $post['id'] );
  }
  
  
  // build the query to find our match
  $sql = 'SELECT id FROM ' . Participants_Db::$participants_table . ' WHERE `' . $check_field . '` LIKE "%\"' . $post[$check_field][0] . '\"%"';
  global $wpdb;
  $result = intval( $wpdb->get_var( $sql ) );
  
  // log the query and results if debugging
  if (PDB_DEBUG) {
    ob_start();
    var_dump($result);
    Participants_Db::debug_log(__FUNCTION__.' match mode: '.$record_match->match_mode().' 
query: '. $wpdb->last_query . ' 
result: '. ob_get_clean() );
  }
  
  /*
   * the $result will be the id of the matched record or 0 if no match
   */
  if ( $result > 0 ) {
    
    $record_match->set_record_id( $result );
    
    // we found a match set status to true
    $match_status = true;
  }
  
  $record_match->set_match_status( $match_status );
}

/**
 * adds a feedback message for signup submissions
 * 
 * this message will be shown if the signup is rejected because it matches an 
 * existing record
 * 
 * @param array $messages
 * @return array
 */
function xnau_check_for_match_feedback( $messages )
{
  // clear the error CSS so the wrong field does not get highlighted
  add_filter( 'pdb-error_css', function ($css) {
    return '';
  } );
  
  // set the feedback message
  $messages['duplicate'] = 'The URL you submitted matches an existing record.';
  
  return $messages;
}

You can download and install this as a plugin:

How to Install a WordPress Plugin from a Gist