我确信我遗漏了一些明显的东西，但是如果这个元值在foreach循环中还不存在，我怎么能让这个函数添加一个元值呢？

你也可以在这里的评论中看到我的一些想法。

php
global $wpdb;
$order_id = $order_data["id"];
$names = array();
$unique_ids = array();
$new_ticket_links = array();
$base_ticket_path = esc_url( home_url( \'/\' ) . \'tickets_download/\');
$ticketidmeta = \'_tribe_ext_pdf_tickets_unique_id\';
$table_name = $wpdb->prefix . "postmeta";
$db_data = $wpdb->get_results("SELECT post_id, meta_key, meta_value FROM $table_name WHERE `meta_value` LIKE $order_id AND post_id <> $order_id");

if (!empty($db_data)) {
foreach ($db_data as $value) {
    $names[] = $value->post_id;
  }
} 
if (!empty($names)) {
foreach ($names as $val) {
  $unique_ids[] = get_post_meta($val, $ticketidmeta, false);

  // If the unique_id isn\'t there, create it
  if ( empty( $unique_ids ) ) {

    /*
     *  I\'ve played with a foreach loop here as well
     *  The results are spotty - doesn\'t seem to work.
     */ 
        // foreach ($names as $val) {

        $unique_id = uniqid( \'\', true );
        // uniqid() with more_entropy results in something like \'59dfc07503b009.71316471\'
        $unique_id = str_replace( \'.\', \'\', $unique_id );

        $unique_id = sanitize_file_name( $unique_id );

        add_post_meta( $val, $ticketidmeta, $unique_id, true );

      // }
  }
}
foreach ($unique_ids as $unique_id) {
  $new_ticket_links[] = $base_ticket_path . implode($unique_id);
  }
}