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Captcha image verification

A good way to avoid automatic form submissions when creating a web form is to add some kind of verification. One of the best ways is to use an image verification, called also captcha. What it does is to dynamically create an image with a random string displayed on it. Then visitor is asked to type that string in a text field and once the form is submitted it checks if the string on the image matches the one inputted by the user. Because there is no easy way to read a text from an image (image recognition) this is a good way to protect your web forms from spammers.
For doing this CAPTCHA I would suggest using a session variable where you store the string generated and displayed on that dynamically generated image.

$text = rand(10000,99999);
$_SESSION["vercode"] = $text;
$height = 25;
$width = 65;

$image_p = imagecreate($width, $height);
$black = imagecolorallocate($image_p, 0, 0, 0);
$white = imagecolorallocate($image_p, 255, 255, 255);
$font_size = 14;

imagestring($image_p, $font_size, 5, 5, $text, $white);
imagejpeg($image_p, null, 80);

Save this code in a file called captcha.php. What this script does is to generate a random number from 10000 to 99999 and then assign it to $_SESSION['vercode']. Then it generates a 25x65 pixels image with black background and white text using size 14. So if you upload that captcha.php file on your web site and open http://www.site.com/captcha.php you will see an image displaying random integer. You will receive a new random integer every time you refresh that page.

Next we need to create our web form.

<form action="submit.php" method="post"> 
Comment: <textarea name="coment"></textarea><br>
Enter Code <img src="captcha.php"><input type="text" name="vercode" /><br>
<input type="submit" name="Submit" value="Submit" />

Above code will create a form with a single textarea box, randomly generated image using the captcha.php script and a text field where you will have to enter the verification code.

All we have to do now is to make the submit.php script which will check if the verification code you enter matches the one that has been randomly generated.

if ($_POST["vercode"] != $_SESSION["vercode"] OR $_SESSION["vercode"]=='') {
echo '<strong>Incorrect verification code.</strong><br>';
} else {
// add form data processing code here
echo '<strong>Verification successful.</strong><br>';

242 Comments to "Captcha image verification"

  • constantin


    May 3, 2010 at 18:53 pm

    i already sent it to php.net too..

    this is respons for
    adam at worldwrestlingmania dot cjb dot net
    06-Dec-2009 04:35(here http://php.net/manual/en/function.imagettftext.php)

    and for all that's using captcha to prevent send information in a form using a robot.

    People you don't need captcha!!!! There is another convenient method , to protect a website for spamming and is much simple:

    Let's consider the 1st page(with the form) and let's say the second ... index.php and receiver.php


    <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><HTML>
    echo('this is the form`s page');
    ?><FORM METHOD=POST ACTION="receiver.php">
    <INPUT TYPE="text" NAME="data"> <INPUT TYPE="submit" VALUE="send!" NAME="send"><BR>
    A form without captcha!!!

    function protectform(){

    if (isset($_SERVER["HTTP_REFERER"]))
    if (!$noterror )
    if ($gethost['host']!==$servername)

    if ($pimp){
    die('Go away hacker!');

    if(isset($_REQUEST['send'])and (trim($_REQUEST['data'])!='') ) echo('We already send to this page this value: '.$_REQUEST['data'].'<br>'); else echo('Please try to fill something in that form!');
    ?><A HREF="index.php">Return to my form</A>

    how to probe it?
    well let's say you already upload it on
    www.example.com/myfolder/ index.php and receiver.php

    so try to digit

    now fill the form's value...and click send.
    now is redirected to receiver.php and you see the right value.

    Let's probe the vulnerability of the script:
    digit again
    now when you see the form press File/Save as from the browser's menu and save it on desktop like index.html

    now try to open with notepad to edit it and change this line:
    <FORM METHOD=POST ACTION="receiver.php">

    to something like this:
    <FORM METHOD=POST ACTION="http://www.example.com/myfolder/receiver.php">

    now save it and double click it from desktop.
    well what you see when you already fill that text form and you send the data to www.example.com/myfolder/ ?

    (For php beginners:www.example.com can be www.banana.com too , i don't know where you will probe this software)

    to all people who said :"php is unsecure" i respond with :
    I am writing scripts from 1992, my opinion like expert is : php rooooooooolez!!! people if you don't know how to write scripts in php try php.net to learn something. me,Constantin

  • jamie


    April 15, 2010 at 03:57 am

    hi guys i have html form page that requires verification tat an email address an valid name were entered, how much to ad your script to it, what would you charge, its contact us form.

  • Someone?


    March 29, 2010 at 03:30 am

    Add header("Content-type: image/jpeg"); to the start of the code, and also imagedestroy($image_p); at the end of the code. The first fixes the jibberish problem and the second prevents memory leakage. Also, you can change the type to png quite easily, and I recommend it as jpeg is a sh!t quality image format.

    You need GD for this. Ubuntu server install: sudo apt-get install php5-gd then sudo /etc/init.d/apache2 restart and tada! the php page will show jibberish, which is actually a jpeg. Use the img tag like it shows, and it works great.

  • Tjerk


    March 2, 2010 at 01:52 am

    I do not get the image back.

    What did i do wrong? use IIS7.5.


  • Meet


    February 22, 2010 at 13:21 pm

    where i will get "add form data processing code here" as per instructions

  • tribhuvan


    January 18, 2010 at 14:51 pm

    plz help me how to insert & retrieve image.

  • Damien Darwick

    Damien Darwick

    January 18, 2010 at 13:18 pm

    How do I modify the code to not have to place it at the top of eh page. I have other code and design that needs to go ahead of that code.

  • JJ


    January 15, 2010 at 22:05 pm


    How can i have some text that when clicked reloads the image?

    Also how can i change the font?


  • jon messing

    jon messing

    December 7, 2009 at 04:53 am

    I have some forms that are general in nature, they just return some information so that people can request a service for a company. But I would like them all to have form captcha. I kind of understand it. How much would you charge to set one of them up and then I can just add the same code to the rest of the forms, there are probably three. I know it would probably take someone about 10 minutes to do ... im just a bit clumsy with this.

    Please let me know.



  • ala barnes

    ala barnes

    November 13, 2009 at 02:19 am

    how do you change the font and also put some letters in the captcha image?

    • alan barnes

      alan barnes

      December 7, 2011 at 12:00 pm

      nevermind, i found out how to add letters, but still need to changed font.

      // Generate a random character string
      function rand_str($length = 32, $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890')
      // Length of character list
      $chars_length = (strlen($chars) - 1);

      // Start our string
      $string = $chars{rand(0, $chars_length)};

      // Generate random string
      for ($i = 1; $i < $length; $i = strlen($string))
      // Grab a random character from our list
      $r = $chars{rand(0, $chars_length)};

      // Make sure the same two characters don't appear next to each other
      if ($r != $string{$i - 1}) $string .= $r;

      // Return the string
      return $string;

      $text = rand_str($length = 6, $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890');
      $_SESSION["vercode"] = $text;

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