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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>';

239 Comments to "Captcha image verification"

  • 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;

  • Aimee duddles

    Aimee duddles

    November 12, 2009 at 01:16 am

    I have no idea what the problem is but this isn't working for me at all. What does it cost for you to do it?

  • jithendrakumar v

    jithendrakumar v

    September 7, 2009 at 22:01 pm

    I am looking a captcha entry url id, and i am interest to this work so,

  • Kiran


    September 7, 2009 at 03:44 am

    The script is very good... but I need to validate the captcha with the user entered value through javascript and need to alert the user when it doesn't match with the captcha value. can u please provide me the code for it..... i tried with ajax but not successful....
    Thanks & Regards,

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