using captcha

To prevent from hacking our website, we can add the captcha code in login form. It can be more secure than only use user name and password for autentication login system.
At this case, this is one of the way how to use captcha code to perform in website security

captcha.php <?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) //uppercase 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"); $rand = _generateRandom(3); $_SESSION['captcha'] = $rand; ImageString($im, 5, 2, 2, $rand[0]." ".$rand[1]." ".$rand[2]." ", ImageColorAllocate ($im, 0, 0, 0)); $rand = _generateRandom(3); ImageString($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); ?>

For example how to implement the code,this case we use captcha code in post a comment form

captcha_demo.php <?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 submission'; } ?>

Download file:
Click here to download file demo captcha code