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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>';

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238 Replies to "Captcha image verification"

Ron October 26, 2007 at 4:56 am | Reply


If I enter a wrong validation number it still allows the form to be sent. What Have I done wrong?
Veselin: hard to guess what could be wrong, but maybe your server does not support SESSIONs or you just made a mistake applying the code on your form :)
jay October 19, 2007 at 11:11 pm | Reply


still doesn\'t execute the SQL
jay October 19, 2007 at 4:11 pm | Reply


i have the captcha image verification working, and i have a redirect once it gets to the submit.php. however, i have a SQL insert statement in between the verification and the redirect, but it\'s not executing the insert statement. any ideas?
Veselin: remove the redirect and see if it will execute the SQL
John October 5, 2007 at 5:20 am | Reply


Nice script, learnt a lot from this example. Thank you for sharing your good work.
HS September 24, 2007 at 2:08 pm | Reply


Steven, the image will appear correctly also in Firefox when you include it in the html image tag. I had the same issue with Opera.
John September 21, 2007 at 10:33 pm | Reply


Very nice script, Planning to use it soon. Thank you.
Stephen Whitehead September 14, 2007 at 1:19 am | Reply


I\'ve uploaded the captcha.php file to my server but when viewing in the firefox browser the image isn\'t displayed, just a series of random symbols, letters and numbers - the image displays fine in IE though. the version of firefox is v2.0.06. Any suggestions on what the issue is?
Viktor September 10, 2007 at 10:47 am | Reply


HI, I use php 4.1.1. and captcha.php returns error message :
Warning: rand(): Invalid range: 10000..99999 in c:\\apache\\htdocs\\colosseum\\captcha.php on line 3

Fatal error: Call to undefined function: imagecreate() in c:\\apache\\htdocs\\colosseum\\captcha.php on line 9

what\'s the probleme?
Sasho Valkanov August 30, 2007 at 11:25 am | Reply


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bigSlim August 27, 2007 at 10:59 am | Reply


Here\'s a weird one that I\'ve searched for in the comments and cannot find... I\'ve installed the captcha code and it appears to be working all fine but for some strange reason im still receiving lots of spam???

What have i done wrong?

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