Redirect or error if the code doesn't exist in code.txt

I have in promotie.php the next form:

<form accept-charset="UTF-8" action="redirect.php">
  <input name="code" type="text" id="code" maxlength="15" />
  <input type="submit" value="Submit"/>
</form>

In redirect.php I have this code:

<?php
header('Location: download.php?code='.$_POST['code']);
?>

Now how can I redirect to promotieerror.php or show a error if the code that was writed by the user doesn't exist in code.txt ?

P.S.: I don't want to use MySQL/Databases.

Thanks in advance for help.