Sam Yong

<?php
session_start();

// note that $_GET['k'] is a static key that you can enter when getting the image.
// generate the key and store into session
$_SESSION['captchasecurity'] = md5(mt_rand().time().$session_hash).$session_hash.md5($_GET['k'].mt_rand());
// create a shorter key for display
$key = dechex(crc32($_SESSION['captchasecurity']));

// disable the client side caching
header('Content-type: image/png');
header('Cache-Control: max-age=0');
header('Expires: '.gmdate('r',time()-3600*24*365));
header('Pragma:');

// randomize the background image
$r = mt_rand(0,3);
$captcha = imagecreatefrompng('img/captcha'.$r.'.png');

$cl = mt_rand(0,50);
$c = imagecolorallocate($captcha, $cl, $cl, $cl);
$line = imagecolorallocate($captcha,233,239,239);

$txtx = imagefontwidth(5) * strlen($key) +10;
$txty = imagefontheight(5);

$timg1 = imagecreate($txtx,$txtx);
$ba = imagecolorallocatealpha($timg1,0, 0, 0,127);
imagefilledrectangle($timg1, 0, 0, $txtx, $txtx, $ba);
imagesavealpha($timg1, true);

$tc = imagecolorallocate($timg1, $cl, $cl, $cl);
imagestring($timg1, 5, 0, $txtx/2 - $txty/2, $key, $tc);

$tca = imagecolorallocatealpha($timg1, 255, 255, 255,127);
$timg = imagerotate($timg1, mt_rand(-5,5), $tca);
imagesavealpha($timg, true);
imagecopy($captcha, $timg, mt_rand(2,22), mt_rand(3,6), 0, $txtx/2 - $txty/2, $txtx, $txtx);

// output the captcha image
imagepng($captcha);

// free up memory
imagedestroy($timg);imagedestroy($timg1);
imagedestroy($captcha);
?>