DISABLED, SORRY :(

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BASH-SCRIPT (execute every 10 minutes via cron):
#!/bin/bash

export REPLYTO="replyto@somedomain.tld"

cd /path/to/temp/or/directory/of/this/script
url="http://www.complang.tuwien.ac.at/knoop/lehre/ws1011/fp185161/?C=M;O=A"
oldsum=$(cat oldsum)

file="content"
content=$(wget -t 1 -T 10 -q -O $file $url)
ret=$?
if [ "$ret" != "0" ]
then
  exit
fi
newsum=$(md5 -q $file)

if [ "$oldsum" != "$newsum" ]
then
  echo "CHANGED!!!"
  for mail in $(cat /path/to/addresses)
  do
    echo -e "From: FP Notifier <from@somedomain.tld>\nTo: $mail\nSubject: [TU Wien] Funk Prog Notify\nsome data on $url has changed!!" | sendmail -t -F"FP Notifier" -ffrom@somedomain.tld
  done
  echo $newsum > oldsum
fi

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PHP-SCRIPT:
$defmail = "eMatrNr@student.tuwien.ac.at";

if (!empty($_POST["addr"]))
{
  $p = explode('@', $_POST["addr"]);
  if (count($p) == 2 && $p[1] == "student.tuwien.ac.at" && is_numeric(substr($p[0], 1)) && strlen($p[0]) == 8)
  {
    $defmail = $_POST["addr"];
    $recipients = explode("\n", trim(file_get_contents(".addresses")));
    $write = false;
    if ($_POST["mode"] == "register")
    {
      $key = array_search($_POST["addr"], $recipients);
      if ($key === false)
      {
        $recipients[] = $_POST["addr"];
        $write = true;
      }
      else
        echo "ALREADY REGISTERED<br /><br />";
    }
    elseif ($_POST["mode"] == "remove")
    {
      $key = array_search($_POST["addr"], $recipients);
      if ($key === false)
        echo "NOT FOUND<br /><br />";
      else
      {
        unset($recipients[$key]);
        $write = true;
      }
    }

    if ($write)
    {
      file_put_contents("/path/to/addresses", implode("\n", array_unique($recipients)));
      echo "DONE<br /><br />";
    }
  }
}

<form action="index.php" method="POST">
  E-Mail: <input type="text" name="addr" size="30" value="<?=$defmail?>" /><br />
  <input type="submit" name="mode" value="register" />
  <input type="submit" name="mode" value="remove" />
</form>