PHP Add Captcha Protection To Web Forms

Posted on in Categories , , , , , , , , , , , last updated September 22, 2009

I own a small business website. However, bots started to abusing my forms such as contact.php. How do I stop bad bots from abusing my site? How do I tell if PHP form is submitted by a person or a script?

You need to use a Captcha, which is nothing but a type of challenge-response test used by you to ensure that the response is not generated by a bot. There are plenty of libraries provided for PHP. I recommend the reCAPTCHA PHP Library, which provides a simple way to place a CAPTCHA on your PHP forms. It can stop bots from abusing it. you need to use the reCAPTCHA API.

Step # 1: Get reCAPTCHA API Library

Visit reCAPTCHA website to sign up for an API key (it is free). Please note down your private and public keys.

Step # 2: Download and Install reCAPTCHA PHP

Download the reCAPTCHA library from Google code repo:
$ cd /tmp
$ wget

Unzip, enter:
$ unzip
Finally, copy recaptchalib.php to the directory where your forms live. For e.g. if your contact.php is at /var/www/html, copy recaptchalib.php as follows:
$ cp /tmp/recaptcha-php-1.10/recaptchalib.php /var/www/html

Step # 3: Test It

Create a php script as follows:

	<title>Sample Email Form</title>
    function checkForm() {
	if (document.forms.myphpform.elements['yname'].value.length == 0) {
		alert('Please enter a value for the "Name" field');
        	return false;
	if (document.forms.myphpform.elements['email'].value.length == 0) {
		alert('Please enter a value for the "Email" field');
        	return false;
	if (document.forms.myphpform.elements['message'].value.length == 0) {
		alert('Please enter a value for the "Message" field');
        	return false;
        return true;
<form action="?done=1" method="post" name="myphpform" onSubmit="return checkForm()"  >
<table border=0>
		<td>Your Name:</td> <td><input type="text" name="yname" size="50" maxlength="50" value="" /></td>
		<td>Your Email:</td> <td><input type="text" name="email" size="50" maxlength="50" value="" /></td>
		<td>Message:</td> <td><input type="text" name="message" size="50" maxlength="50" value="" /></td>
		<td>Are you a human being?</td>
$publickey = "YOUR-PUBLIC-KEY";
$privatekey = "YOUR-PRIVATE-KEY";
$resp = null;
$error = null;
# are we submitting the page?
if ($_POST["submit"]) {
  $resp = recaptcha_check_answer ($privatekey,
  if ($resp->is_valid) {
	$to="[email protected]";
	$subject="Feedback from";
        $body=" Message via webform: 
Name: " .$_POST["yname"] . "\n
Email: " .$_POST["email"] . "\n
Message: " .$_POST["message"] . "\n";
        /*  send email */
	echo "<p>Email sent!</p>";
  } else {
     	echo "Sorry cannot send email as you've failed to provide correct captcha! Try again...";
echo recaptcha_get_html($publickey, $error);
		<td><input type="submit" name="submit" value="submit" /></td>

Sample Output:

Fig.01: PHP Captcha in Action
Fig.01: PHP Captcha in Action

You can see working captcha example by visiting this url.

Further readings:

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

26 comment

    1. Some guys are just stupid “Mr. Someone” I’m writing this to you, if you think you better, then contribute what you have than criticizing what the good Samaritan is doing. However, I much appreciate the author’s works. God bless him.
      A word to the Author: Keep your head up man, and thank you for your lovely work, God bless you abundantly.

      1. hey ‘mr. someone’… are you a php hard coder? then why don’t you create one for yourself…and it’s obvious that the reason you are here is because your looking also for a solution in your captcha problem… that shows you can’t do it urself… moron!… advice: dont criticize someone’s good intention, look at yourself first.. i dont think your good at php and one more thing… i know im better that you asshole. haha… hey author, good contribution there keep it up!! nice work

  1. I just came across this article: Feel I want to comment on @someone ‘s comment.
    ** You’re an ASS**
    The Author took the time to write a good solid article. If you don’t approve or like.
    At least give constructive comment


  2. hy,
    i installed captcha on one of my forms and when i tipe the wrong word in the box and i click on send button it drives me to a blank page witch tells me that captcha is incorrenct and i have to click back.
    all i want is to appear like on this page to reload only the captcha, not the entire page, and to remain on the same page.
    please help me :(

  3. Adding a captcha to blog comment form and on other registration can minimize the usage software to spam your website and because of that it can lessen your job. Thanks for the great tutorials.

  4. This would be good for embeding in existing embedded webpages if it had a few improvements.
    A couple of suggestions.
    -Give it a real “from” email address instead of having appear.
    -Included a clear form button. (easy enough even for me to add)
    -Instead of having it reload the page and adding a “?done=1” have it do the following
    a.replace the recapatcha/clear/submit buttons with a green box saying “successfully sent” without refreshing the page.
    b.If a field is empty have a popup dialog box saying fill in such and such.

  5. i use it and it works well
    but i have a big problem :(
    when i insert a wrong value in captcha fld and press submit , no error will show and whole form works without any error !

  6. [email protected] Online with Source Codes says:

    Thanks VIVEK GITE …. Awesome post of you with proper source code and outputs.. I was searching for this from near about 2 years. I found official site but can;t understand code.. This helped me a lot… R u from India ???

  7. Hi,
    Nice article, I learned who to build my site using MS Expressions Web 4, but there is no way I could get reCAPTCHA to work until reading and using your code.
    But how do I hide my private key number? Any help with hiding my private key would be greatly appreciated.

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