Creating a Dynamic Vertical Gradient in PHP

validate.php

<?php

// provide a generic 404 error if...
if (

  // FROM/START color does not exist
  !array_key_exists('from', $_GET) ||

  // FROM/START color is not valid hex
  !preg_match('/^[a-fA-F\d]{6}$/', $_GET['from']) ||

  // TO/END color does not exist
  !array_key_exists('to', $_GET) ||

  // TO/END color is not valid hex
  !preg_match('/^[a-fA-F\d]{6}$/', $_GET['to']) ||

  // HEIGHT does not exist
  !array_key_exists('height', $_GET) ||

  // HEIGHT is not numeric
  !is_numeric($_GET['height']) ||

  // Be sure to set your minimum and maximum heights that you'll be
  // using here. If you need a 300px gradient, you'll need to up the
  // value to 300. But be sure to use a limit! A user who inputs
  // 9,999,999 will destroy your server's memory and bandwidth.

  // HEIGHT is too small
  $_GET['height'] < 4 ||

  // HEIGHT is too large
  $_GET['height'] > 64
) {

  // 404 header
  header('HTTP/1.1 404 Not Found');

  // text document
  header('Content-Type: text/plain; charset=utf-8');

  // generic 404 error message
  exit('Move along. Nothing to see here.');
}

?>