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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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244 Comments to "Captcha image verification"

  • Joe

    Joe

    August 13, 2007 at 01:01 am

    Great script, Thanks

  • Gil

    Gil

    August 10, 2007 at 04:31 am

    Thank you so much! it works!

  • Astiak

    Astiak

    August 6, 2007 at 21:40 pm

    $font_size = 14;

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

    Otherwise, great script :).

  • carmelo portugues

    carmelo portugues

    July 28, 2007 at 02:58 am

    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

    Jeff

    July 24, 2007 at 16:14 pm

    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! :)

  • niikii

    niikii

    July 16, 2007 at 22:08 pm

    i saw a X , can't see image. any idea?

  • Rion Massie

    Rion Massie

    July 16, 2007 at 18:41 pm

    Hey thanks for this bit of code. It works perfectly!

  • Dave

    Dave

    July 5, 2007 at 04:55 am

    I need you to add this script to my contact form for me.
    Not sure how to do it. I can easily cut and paste and do a FTP upload of the script to my server.
    here is my current form.
    www.listedgreen.com/contact.html
    I need it to work with my current PHP form that has a field required script too. See my code.
    Is it PayPAL time?
    -----------------------------
    Veselin: We should change contact.html to contact.php. Please, send me FTP login info and once I confirm that it can be done you can paypal the money.

  • TheGeekster

    TheGeekster

    July 4, 2007 at 23:51 pm

    Hi I am trying using this script in a guest book. Before I put this in, it ran www.*****.com/tg.pl. How do I edit submit.php to run this?? Thanks! Awesome script BTW!!!

  • bailey

    bailey

    July 3, 2007 at 06:59 am

    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

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