discord/index.php

<?php require 'connect_discord_oauth.php'; $user_discordID = getUserId($_GET['code']); ?>
<script type="text/javascript">
// set parent to #discord_id
let parent = window.parent.document.getElementById('discord_id');

// set parent value to users discord client id
parent.value = <?php echo $user_discordID; ?>

// if value of input#discord_id = code...
if(parent.value == <?php echo $user_discordID; ?>) {

  // close window
  close();

}
</script>