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Captcha image verification Posted in PHP Tutorials | 234 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>';

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234 Replies to "Captcha image verification"

brandon December 18, 2008 at 11:05 pm | Reply


nice script.. thanks
whataboutchris December 14, 2008 at 3:25 am | Reply


GREAT SCRIPT! I have 3 big questions.
1. How do I submit the form contents to an email?
2. How do you go to a separate page on successful submission?
3. How do I place the error code on the same page as the form?
Shive Prakash Dhaker December 6, 2008 at 7:40 am | Reply


Thanks a lot.It's so simple and nice Script.
developer December 1, 2008 at 12:35 pm | Reply


Incorrect verification code.this message coming always my page while populating..what is this how to resolve this?
Veselin: most probably your server does not support SESSION variables. Also make sure your browser supports cookies.
Newer November 19, 2008 at 1:34 pm | Reply


Hi , what about if i open this script in twwo browsers , and first i try ti enter code in first then in second ? I suppose that script will reset image and it write error as code do not conside ,while second browser everything will be ok , is such captcha good ?
Man0r November 16, 2008 at 4:15 pm | Reply


Thank you!! Its work PERFECT :))))

Man0r :)
awais November 15, 2008 at 10:47 am | Reply


Hello Awais here.
i used this code in my login page but the doesn't appear on the login page.what should i do now?
Veselin: most probably you did not put the code on the right place
Manor November 14, 2008 at 8:52 pm | Reply


Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/man0r/domains/israel-fm.co.il/public_html/tests/mailto.php:2)
Any ideas on how to fix it?
Veselin: please make sure you put session_start() at the very top of your file
vaibhav November 13, 2008 at 8:53 am | Reply


This is one of the best code u have write for the captcha image validation..
Thank fot that
do u have code for captcha image with numbers and alphabates.
Veselin: you should replace
$text = rand(10000,99999);
$text = strtoupper(substr(str_shuffle('0123456789abcdefghjkmnpqrstuvwxyz'), 0, 4));
Gabe September 13, 2008 at 6:36 am | Reply


I can't get this code to bounce an invalid number. It will tell me that the number is wrong, but the form will still submit.
Veselin: the code that processes the form data should be within the IF statement where echo '<strong>Incorrect verification code.</strong><br>'; is.

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