You don’t need create multiple accounts for multiple domains, all you need to do is check the checkbox right under your domain name.

Next, you get to this page with your API keys. Note those API keys since you are going to use them in your implementation. Obviously, mine are covered right now :)

Anyways, then you can either click “reCAPTCHA plugins and libraries” and figure out how to implement it yourself or read the rest of this post :) They do have explanations on ASP.Net and more if you use a different language on your website. You can have different programming languages on different sites and still use the same API keys by the way.

Next, you need to download the library from here and then upload it to your server. Now open the page where you post something, anything really.. that you want the reCaptcha to show up and enter the following code;

<?php
require_once('recaptchalib.php');
$publickey = "Your public API key"; // you got this from the signup page
echo recaptcha_get_html($publickey);
?>

Note: You can put the first two lines in your global include file and then use the last line for the captcha to show up anywhere.

Now, open the page where you get the data from your form. Before you do any checks (e.g. Check empty fields, or email address validation), you want to check the captcha.

So include the following code in the beginning of your file;

<?php
require_once('recaptchalib.php');
$privatekey = "Your private API key";
$resp = recaptcha_check_answer ($privatekey,
 $_SERVER["REMOTE_ADDR"],
 $_POST["recaptcha_challenge_field"],
 $_POST["recaptcha_response_field"]);
?>

And where you want the check to happen include the following code;

<?php
if (!$resp->is_valid) {
 echo "The reCAPTCHA wasn't entered correctly. Go back and try it again.";
}
?>

That’s it. You should be able to run the code now. Let me know, if you run into any problems and I will try to help.