Parse dedeCMS captcha implementation code

<?php
$rndstring = '';
for($i=0; $i<4; $i++) $rndstring .= chr(mt_rand(65,90));
// If the support GD , the drawing 
if(function_exists("imagecreate"))
{
 //Firefox Part of the situation will require multiple requests, 5 Refreshing the page in seconds will not change session
 $ntime = time();
 if(empty($_SESSION['dd_ckstr_last']) || empty($_SESSION['dd_ckstr']) || ($ntime - $_SESSION['dd_ckstr_last'] > 5))
 {
  $_SESSION['dd_ckstr'] = strtolower($rndstring);
  $_SESSION['dd_ckstr_last'] = $ntime;
 }
 $rndstring = $_SESSION['dd_ckstr'];
 $rndcodelen = strlen($rndstring);
 // Create the image and set the background color 
 $im = imagecreate(50,20);
 ImageColorAllocate($im, 255,255,255);
 // Background line 
 $lineColor1 = ImageColorAllocate($im,240,220,180);
 $lineColor2 = ImageColorAllocate($im,250,250,170);
 for($j=3;$j<=16;$j=$j+3)
 {
  imageline($im,2,$j,48,$j,$lineColor1);
 }
 for($j=2;$j<52;$j=$j+(mt_rand(3,6)))
 {
  imageline($im,$j,2,$j-6,18,$lineColor2);
 }
 // Picture frame 
 $bordercolor = ImageColorAllocate($im, 0x99,0x99,0x99);
 imagerectangle($im, 0, 0, 49, 19, $bordercolor);
 // The output text 
 $fontColor = ImageColorAllocate($im, 48,61,50);
 for($i=0;$i<$rndcodelen;$i++)
 {
  $bc = mt_rand(0,1);
  $rndstring[$i] = strtoupper($rndstring[$i]);
  imagestring($im, 5, $i*10+6, mt_rand(2,4), $rndstring[$i], $fontColor);
 }
 header("Pragma:no-cache/r/n");
 header("Cache-Control:no-cache/r/n");
 header("Expires:0/r/n");
 // Output a specific type of image format, priority is  gif -> jpg ->png
 if(function_exists("imagejpeg"))
 {
  header("content-type:image/jpeg/r/n");
  imagejpeg($im);
 }
 else
 {
  header("content-type:image/png/r/n");
  imagepng($im);
 }
 ImageDestroy($im);
 exit();
}
else
{
 // Does not support GD , output only letters  ABCD
 $_SESSION['dd_ckstr'] = "abcd";
 $_SESSION['dd_ckstr_last'] = '';
 header("content-type:image/jpeg/r/n");
 header("Pragma:no-cache/r/n");
 header("Cache-Control:no-cache/r/n");
 header("Expires:0/r/n");
 $fp = fopen("data/vdcode.jpg","r");
 echo fread($fp,filesize("data/vdcode.jpg"));
 fclose($fp);
 exit();
}
?>