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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.

<?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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238 Comments to "Captcha image verification"

  • kaushalya

    kaushalya

    August 20, 2011 at 19:50 pm

    I have added captcha.php & Submit.php to my website.

    It is looking perfact in front end but there is no verfication being performed by code please see... http://topcafirms.com/sales#contact

    I am non technical...I am not able to understand "// add form data processing code here" in submit.php what do i need to add here in this file.

    There is separate file for mail which contains lot of code and I am not able to understand to add here....is it

    @mail($emailid, $subject, $msg, $headers);
    or somthing else to be added....

  • mustafa khan

    mustafa khan

    August 9, 2011 at 16:08 pm

    i want to convert verification code into text format
    PLZ
    anyone tell me software
    thanking you

  • Stephan Dorkes

    Stephan Dorkes

    July 27, 2011 at 10:35 am

    How do you identify an image captcha if it's only a single picture?

  • Umar

    Umar

    June 19, 2011 at 10:01 am

    Thankz, it works nicely

  • Tejitha Chowdary

    Tejitha Chowdary

    May 28, 2011 at 14:24 pm

    I would like to know about how "Image Captcha" is used to prevent the Brute Force Hack after giving three wrong passwords?

  • dino

    dino

    May 3, 2011 at 05:46 am

    a, ok
    is this the same?

  • Wiyono

    Wiyono

    April 11, 2011 at 12:40 pm

    how to refresh images captcha?

  • Nikolay

    Nikolay

    April 6, 2011 at 12:00 pm

    Nice and simple captcha that does the job as it should! Good job to the author!

  • John

    John

    March 12, 2011 at 15:00 pm

    I request to verify my google account.I want to insert the code that was sent to my phone.

  • tamil

    tamil

    March 9, 2011 at 13:14 pm

    This captcha coding very useful of my project.

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