How to Implement the CAPTCHA with PHP and GD

So, you have a public submission form on your website (contact page, forum submission):

Contact us (Post new message):

<form method="post" action="">
<table bgcolor="#CCCCCC">
<tr><th>Contact us (Post new message):</th></tr>
<tr><td><textarea cols="30" rows="5"></textarea></td></tr>
<tr><th><input type="submit" value="Submit"></th></tr>
</table>
</form>

and need to prevent spam auto-submitters. Common way to do this is to implement CAPTCHA - an image with randomly generated string:

Simple and quick PHP solution:

1. Make a PHP script which will generate the image:

<?php
session_start();
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");
header("Cache-Control: no-store, no-cache, must-revalidate");
header("Cache-Control: post-check=0, pre-check=0", false);
header("Pragma: no-cache");

function _generateRandom($length=6)
{
    $_rand_src = array(
        array(48,57) //digits
        , array(97,122) //lowercase chars
//        , array(65,90) //appercase chars
    );
    srand ((double) microtime() * 1000000);
    $random_string = "";
    for($i=0;$i<$length;$i++){
        $i1=rand(0,sizeof($_rand_src)-1);
        $random_string .= chr(rand($_rand_src[$i1][0],$_rand_src[$i1][1]));
    }
    return $random_string;
}

$im = @imagecreatefromjpeg("captcha.jpg");
$white = ImageColorAllocate ($im, 255, 255, 255);
$black = ImageColorAllocate ($im, 0, 0, 0);
$rand = _generateRandom(3);
$_SESSION['captcha'] = $rand;
ImageString($im, 5, 2, 2, $rand[0]." ".$rand[1]." ".$rand[2]." ", $black);
$rand = _generateRandom(3);
ImageString($im, 5, 2, 2, " ".$rand[0]." ".$rand[1]." ".$rand[2], ImageColorAllocate ($im, 255, 0, 0));
Header ('Content-type: image/jpeg');
imagejpeg($im,NULL,100);
ImageDestroy($im);
?>

2. Add the following line at the top of the page where you need to implement CAPTCHA:

<?php session_start() ?>

3. Add the following line to check is CAPTCHA entered by visitor valid, before the line where you will proceed submitted message:

<?php
if($_SESSION["captcha"]==$_POST["captcha"])
{
//CAPTHCA is valid; proceed the message: save to database, send by e-mail ...
}
?>

4. Finaly add the CAPTCHA to the form:

CAPTCHA:
(antispam code, 3 black symbols)

<?php session_start() ?>
<form method="post" action="">
<table bgcolor="#CCCCCC">
<tr><th>Contact us (Post new message):</th></tr>
<tr><td><textarea cols="30" rows="5" name="message"></textarea></td></tr>
<tr><td align="center">CAPTCHA:<br>
    (antispam code, 3 black symbols)<br>
    <table> <tr> <td> <img src="captcha.php" alt="captcha image"> </td> <td> <input type="text" name="captcha" size="3" maxlength="3"> </td> </tr> </table>
</td></tr>
<tr><th><input type="submit" value="Submit"></th></tr>
</table>
</form>
<?php
if(isset($_POST["captcha"]))
if($_SESSION["captcha"]==$_POST["captcha"])
{
    //CAPTHCA is valid; proceed the message: save to database, send by e-mail ...
    echo 'CAPTHCA is valid; proceed the message';
}
else
{
    echo 'CAPTHCA is not valid; ignore the submission';
}
?>

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