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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>';
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240 Comments to "Captcha image verification"

  • Jeff


    July 24, 2007 at 16:14 pm

    Wow, thank you for the help getting this to work with my guest book. It was a small fee to pay, and you got the work done fast! :)

  • niikii


    July 16, 2007 at 22:08 pm

    i saw a X , can't see image. any idea?

  • Rion Massie

    Rion Massie

    July 16, 2007 at 18:41 pm

    Hey thanks for this bit of code. It works perfectly!

  • Dave


    July 5, 2007 at 04:55 am

    I need you to add this script to my contact form for me.
    Not sure how to do it. I can easily cut and paste and do a FTP upload of the script to my server.
    here is my current form.
    I need it to work with my current PHP form that has a field required script too. See my code.
    Is it PayPAL time?
    Veselin: We should change contact.html to contact.php. Please, send me FTP login info and once I confirm that it can be done you can paypal the money.

  • TheGeekster


    July 4, 2007 at 23:51 pm

    Hi I am trying using this script in a guest book. Before I put this in, it ran www.*****.com/tg.pl. How do I edit submit.php to run this?? Thanks! Awesome script BTW!!!

  • bailey


    July 3, 2007 at 06:59 am

    what do you do when the image will not show?
    Veselin: make sure that your server supports GD library and try to open www.yourwebsite.com/captcha.php to see the error message

  • Gia


    June 23, 2007 at 04:30 am

    I love the script and was able to add it to my site. It works well. However, I would like to have it so that if they put in the wrong number "captcha" the email will not be sent. Any suggestions?
    Veselin: you need to add the captcha verification if() statement in your mail sending script

  • Johnny


    June 20, 2007 at 12:39 pm

    is it possible to add an alert for verifying the verification code instead of it navigating to another page?
    Veselin: using AJAX it is possible to do a captcha verification
    Johny: is it possible if u could post the codes for using AJAX to do the verification?
    Veselin: we do not have the code but for a small fee can do it for you

  • William


    June 13, 2007 at 09:28 am

    Very nice script. But how to add just one captcha verification line in an existing form?

  • Nancy


    June 11, 2007 at 16:13 pm

    When get an error message when running the script:
    "Fatal error: Call to undefined function: imagecreate() in testnew.php on line 9"
    Any suggestions ?
    Veselin: most probably your PHP does not have GD library installed or imagecreate() function is unsupported.

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