Captchas

<?php
// safe this to a separate file image.php
session_start();
$letters=substr(str_shuffle("ACDJKMNOPSTVWXYZ"),0,4);
$image = imagecreate(40,16);
imagecolorallocate($image, 255, 255, 255);
imagecolorallocate($image, 0, 0, 0);
imagecolorallocate($image, rand(50,175), rand(125,225), rand(125,225));
imagecolorallocate($image, rand(125,225), rand(50,175), rand(125,225));
imageline($image, 0, rand(9,14), 40, rand(2,7), 2);
imagestring($image, 5, 3, 0, " " . $letters[0] . " " . $letters[1], 1);
imageline($image, 0, rand(2,7), 40, rand(9,14), 3);
imagestring($image, 5, 3, 0, $letters[2] . " " . $letters[3], 1);
$_SESSION["md5"] = md5($letters[2] . $letters[0] . $letters[3] . $letters[1]);
header('Content-type: image/png');
imagepng($image);
 
// usage example:
<img src="./image.php"/>
<label>type in the code:</label>
<input name="c" type="text"/>
 
// validation example:
if (isset($_POST['c']) { $captcha = stripslashes(trim(htmlspecialchars($_POST['c'])));
} else {
$captcha = false;
}
if ( $captcha && isset($_SESSION["md5"])&&$_SESSION["md5"]==md5( strtoupper($captcha) ) ) {
// handle POST vars here
print "Thanks for your submission";
}
?>