« File uploading with PHPPut watermark on images using PHP »

Captcha image verification Posted in PHP Tutorials | 238 Comments
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>';

Do you know PHP / HTML / CSS / JS well?

Write tutorial on a topic you are good in and become a trusted PHP jabber! Share your experience with millions of other webmasters visiting our website. Contact us for more information how to become a contributor.

238 Replies to "Captcha image verification"

bailey July 3, 2007 at 6:59 am | Reply


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 4:30 am | Reply


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 | Reply


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 9:28 am | Reply


Very nice script. But how to add just one captcha verification line in an existing form?
Nancy June 11, 2007 at 4:13 pm | Reply


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.
Mira May 19, 2007 at 5:40 am | Reply


I have tried to use this, but with browser Opera 9.20, the session gode only shows up as cryptic code at the top, form is displayed but no image.
Anyone have an idea why this is happening?
In IE7, only the image is displayed but the form is not.
Mark Jackson May 13, 2007 at 1:58 am | Reply


How do I get the form to email me the results?
Veselin: you can combine this tutorial with <a href='http://www.phpjabbers.com/make-contact-form-and-send-email-in-php-php21.html'>contact form tutorial</a>
jerry jennings May 11, 2007 at 10:51 pm | Reply


Please give us a price on getting bulkconveyors.com/order.html working with captcha
It now works but it transfers data with any number in the verify box
Robert May 9, 2007 at 7:02 pm | Reply


WOW. This script is superb.
angie May 8, 2007 at 12:12 pm | Reply


I have used your scripot and all is working well, however is there a way to make the email address show in the from feild of the incoming email? at the moment i have to copy and past efrom the email feild of the form, would be great if i could just hit reply?
Veselin: this depends on your server configuration. However this <a href='http://www.phpjabbers.com/make-contact-form-and-send-email-in-php-php21.html'>contact form example</a> is done to send emails having the FROM field set to senders email

this part here
$mailheader = "From: ".$_POST["email"]."\r\n";
$mailheader .= "Reply-To: ".$_POST["email"]."\r\n";

Please be polite and helpful and do not spam or offend others. We promise you will be treated the same way.

Log in your free account or if you still haven't joined you can create your free account now.

Posting tip:
if you use code in your comments please put it in these tags [php], [sql], [css], [js]
PHP code example: [php] echo date("Y-m-d"); [/php]

Thank you,
~ PHPJabbers team ~