Edit: Updated code is posted here: http://www.superiorwebsys.com/blog/92/Make_PHP_CAPTCHA_more_secure/

In last article I explained what CAPTCHA is and why it is used (you can find more information about CAPTCHA here). Here I will provide PHP code that crates and uses CAPTCHA..

<?
session_start();
header("Content-type: image/png");
$_SESSION["secureNumber"]="";
//Size of the image Width, Height
$im = imagecreate(105, 20);        
//Set background color
imagecolorallocate($im, 167, 218, 239); 
$grey = imagecolorallocate($im, 128, 128, 128);
$black = imagecolorallocate($im, 0, 0, 0);        
// You can replace fornt by your own
$font = 'LHANDW.TTF';        
for($i=0;$i<=5;$i++) {
    $numb=rand(0, 9);
    $_SESSION["secureNumber"].=$numb;
    $angle=rand(-25, 25);
    imagettftext($im, 9, $angle, 8+15*$i, 13, $black, $font, $numb);    
    // Add shadow to the text    
    imagettftext($im, 9, $angle, 9+15*$i, 14, $grey, $font, $numb);    
}
imagepng($im);
imagedestroy($im);
?>

How to use this code

1. Display CAPTCHA and input field

<table border="0" cellpadding="0" cellspacing="0">
<tr>
    <td valign="top"><input name="secretNumber"  value="" maxlength="6" /></td>
    <td valign="bottom">
        <img src="/blog/posts/55/secureImage.php" alt="CAPTCHA" />
    </td>
</tr>
</table>

2. Check if correct number was entered

<?
if(!empty($_POST["secretNumber"]) &&
    $_SESSION["secureNumber"]==$_POST["secretNumber"])
{
    //Correct number is entered
}
{
    //Display error message
}
?>

Using CAPTCHA will help to solve refresh problem (You can read about refresh problem here)

In following posts I will provide C# code for CAPTCHA.

Michael Pankratov