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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>';

238 Comments to "Captcha image verification"

  • pong


    July 14, 2008 at 18:57 pm

    Hiya, I'm also getting weird text each time I load the captcha script (the php file) by itself. I'm also wondering if the integer type $text is typecasted into a string when it is used as a parameter in imagestring();.

  • Daniel Bithell

    Daniel Bithell

    July 10, 2008 at 21:22 pm

    Fatal error: Call to undefined function imagecreate() in /home/******/public_html/captcha.php on line 9

    Any ideas on how to fix?
    Veselin: this means that PHP version on your server does not support the image functions used to create the image

  • Hanshi


    June 26, 2008 at 00:18 am

    When i test out the captcha.php file i get a page full of wierd weddings characters...but it does change everytime i refresh. Can someone help me out? thanks!
    Veselin: please send URL to your web page

  • shubhangi


    June 20, 2008 at 14:37 pm

    how to upload file on web site

  • Al Teal

    Al Teal

    June 19, 2008 at 16:07 pm

    Getting this error:

    Fatal error: Call to undefined function imagecreate() in /var/www/pmiapp/captcha.php on line 9
    Veselin: this means that your PHP version does not support the GD functions used to generate image

  • Florian


    June 18, 2008 at 16:01 pm

    i am using it for guestbook entries. It creates image, if not entering the number the message "Incorrect verification code" is shown, but adds entry anyway? Any ideas?
    Veselin: most probably the insert guestbook entry is not in the right place. It should be where you have this

    // add form data processing code here
    echo '<strong>Verification successful.</strong><br>';

  • Sandy


    June 16, 2008 at 17:56 pm

    is there a chart of the different color codes for this? that i can look at ?
    Veselin: these are RGB color code values

  • Sandy


    June 16, 2008 at 17:31 pm

    How do we get the black background to blue?
    Veselin: change this
    imagecolorallocate($image_p, 0, 0, 0);
    imagecolorallocate($image_p, 70, 10, 220);

  • tom


    June 12, 2008 at 22:43 pm

    For doing this CAPTCHA I would suggest using a session variable where you store the string generated and displayed on that dynamically generated image.

    can you explain this instruction in detail? what is a session variable? store the string? where do i get the dynamically generated image? the referenced website (http://www.site.com/captcha.php) didn't work...

    thank you!

  • Elaine


    April 24, 2008 at 14:41 pm

    This code works great, but one thing i would like it to do if anyone can tell me how to do this, i would like the error message to display next to the verification box instead of at the top of the page , is that possible?
    Veselin: you need to have your form on PHP based web page and the verification to be on top of it and then print the error message where your box is. For a small fee we can make your contact form on PHP page and also make the message display next to the captcha box. Email us if you are interested.

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