Creating a Dynamic Vertical Gradient in PHP

dynamic-vertical-gradient.php

<?php

// Validate inputs.
if (
  !array_key_exists('from', $_GET) ||
  !preg_match('/^[a-fA-F\d]{6}$/', $_GET['from']) ||
  !array_key_exists('to', $_GET) ||
  !preg_match('/^[a-fA-F\d]{6}$/', $_GET['to']) ||
  !array_key_exists('height', $_GET) ||
  !is_numeric($_GET['height']) ||
  $_GET['height'] < 4 ||
  $_GET['height'] > 64
) {
  header('HTTP/1.1 404 Not Found');
  header('Content-Type: text/plain; charset=utf-8');
  exit('Move along. Nothing to see here.');
}

// Calculate start RGB.
$start = Array();
for ($x = 0; $x < 3; $x++)
  array_push($start,
    stripos('0123456789abcdef',
      substr($_GET['from'], $x * 2, 1)
    ) * 16 +
    stripos('0123456789abcdef',
      substr($_GET['from'], $x * 2 + 1, 1)
    )
  );

// Calculate end RGB.
$end = Array();
for ($x = 0; $x < 3; $x++)
  array_push($end,
    stripos('0123456789abcdef',
      substr($_GET['to'], $x * 2, 1)
    ) * 16 +
    stripos('0123456789abcdef',
      substr($_GET['to'], $x * 2 + 1, 1)
    )
  );

// Calculate steps.
$height = $_GET['height'];
$step = Array();
for ($x = 0; $x < 3; $x++)
  array_push($step, ($end[$x] - $start[$x]) / $height);

// Generate GIF.
header('Content-Type: image/gif');
$gradient = imagecreatetruecolor(1, $height);
for ($x = 0; $x < $height; $x++)
  imagesetpixel(
    $gradient,
    0, $x,
    imagecolorallocate(
      $gradient,
      $start[0] + $step[0] * $x,
      $start[1] + $step[1] * $x,
      $start[2] + $step[2] * $x
    )
  );

imagegif($gradient);
imagedestroy($gradient);

?>