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Captcha image verification Posted in PHP Tutorials | 232 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.

<?php 
session_start();
$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" />
</form>


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.

<?php 
session_start();
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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232 Replies to "Captcha image verification"

Stephen Whitehead September 14, 2007 at 1:19 am | Reply

0

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

0

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

0

If you have any additions, suggestions or modifications for this example please send an email to info@phpjabbers.com or use the form below.
bigSlim August 27, 2007 at 10:59 am | Reply

0

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?
Kelvin Baggs August 19, 2007 at 10:23 am | Reply

0

Hi
I have got your Captcha code working just great on my first web site and when I copied the code to the 2nd site, the image is display ok, but it fails in the comparision test. I have tried to output $_SESSION[\"vercode\"] outside the captcha.php file but it seems like it is not set.
1st Site: http://www.******.com/admin/email/contact.php
2nd Site: http://www.******.com.au/newsite/admin/email/contact.php
Thanks for any help in advance.
regards
-------------------
Veselin: do you have both websites on the same hosting account? It is possible that website 2 does not support SESSIONs
Joe August 13, 2007 at 1:01 am | Reply

0

Great script, Thanks
Gil August 10, 2007 at 4:31 am | Reply

0

Thank you so much! it works!
Astiak August 6, 2007 at 9:40 pm | Reply

0

$font_size = 14;

The maximum font size supported by imagestring() is 5 unless you load a font using imageloadfont().

Otherwise, great script :).
carmelo portugues July 28, 2007 at 2:58 am | Reply

0

just created a non-profit organization, and would like to have a captcha on my donation page or index page..would like to know how much is your fee...thank you
Jeff July 24, 2007 at 4:14 pm | Reply

0

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! :)


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