How to dynamically generate random pictures of specified size in PHP_php tips

<&#63;php
$image_width = 100;
$image_height = 100;
$image_str = '';
if (isset($_GET['w']))
{
  $image_width = intval($_GET['w']);
}
if (isset($_GET['h']))
{
  $image_height = intval($_GET['h']);
}
if (isset($_GET['s']))
{
  $image_str = $_GET['s'];
}
$img = imagecreate($image_width, $image_height);
$color = imagecolorallocate($img, mt_rand(157,255), mt_rand(157,255), mt_rand(157,255));
imagefilledrectangle($img, 0, $image_height, $image_width, 0, $color);
$step = mt_rand(15, 30);
$start = mt_rand(0, $step);
$color = imagecolorallocate($img, mt_rand(200,255), mt_rand(200,255), mt_rand(200,255));
imagesetthickness($img, mt_rand(3, 10));
if ($image_height > $image_width)
{
  for ($i=$start; $i<$image_height * 2; $i+=$step)
  {
    imageline($img, 0, $i, $i, 0, $color);
  }
}
else
{
  for ($i=$start; $i<$image_width * 2; $i+=$step)
  {
    imageline($img, $i, 0, 0, $i, $color);
  }
}
if ($image_str != '')
{
  $black = imagecolorallocate($img, 0, 0, 0);
  imagestring($img, 12, 5, 5, $image_str, $black);
}
header('Content-type:image/png');
imagepng($img);
imagedestroy($img);