subscribe.php

<?php
// $modx->log($modx::LOG_LEVEL_ERROR,'Testing my custom hook.');
$email = $hook->getValue('subscribe-email');

$base_path = $modx->getOption('base_path');
$fp = fopen($base_path . 'subscribe/subscribe_db.txt', 'a+');
$matches = false;
$lines = file($base_path . 'subscribe/subscribe_db.txt'); // reads a file into a array with the lines

foreach ($lines as $line) {
  if (strstr($line, $email)) {
    $matches = true;
  } 
}

if(!$matches) {
  $text = $email . ",\n";
  fwrite($fp, $text);
}

unsubscribe.php

<?php
if($email = isset($_GET['unsubscribe'])) {
    $email = $_GET['unsubscribe'];

    
  $filename = $base_path . 'subscribe/subscribe_db.txt';
  $lines = file($filename); // reads a file into a array with the lines
  $output = '';

  foreach ($lines as $line) {
    if (!strstr($line, $email)) {
      $output .= $line;
    } 
  }

  // replace the contents of the file with the output
  file_put_contents($filename, $output);
	fclose($filename);
  echo "<p>Вы отписаны от рассылки!</p>";
}