CAPTCHA is an acronym for the expression "Completely Automated Public Turing test to Tell Computers and Humans Apart" (a fully automated public Turing test for differentiation between computers and humans): a cognitive challenge test, used as an anti-human toolspam, pioneered at Carnegie-Mellon University. As the test is administered by a computer, in contrast to the standard Turing test which is administered by a human, this test is actually correctly described as a reverse Turing test.
An example of captcha with reCAPTCHA
Example HTML form with snippet:
<form method="post" action="verify.php">
<?php
require_once('recaptchalib.php');
$publickey = "your_public_key"; // cadastre-se no link informado acima
echo recaptcha_get_html($publickey);
?>
<input type="submit" />
</form>
Chunk to validate from the server side:
<?php
require_once('recaptchalib.php');
$privatekey = "your_private_key"; // observe a imagem em anexo.
$resp = recaptcha_check_answer ($privatekey, $_SERVER["REMOTE_ADDR"], $_POST["recaptcha_challenge_field"], $_POST["recaptcha_response_field"]);
if (!$resp->is_valid) {
// seu fluxo caso o usuário tenha errado o captcha
} else {
// seu fluxo caso o usuário tenha acertado o captcha
}
?>
Additional information after you register to use the API:
Additional source and documentation
A curiosity about reCAPTCHA (Offtopic, by Wikipedia)
reCAPTCHA provides, for subscribed websites, images of
words that optical character recognition (OCR) software does not
was able to identify. These sites subscribed (which their
purposes are not usually related to the project’s help
digitization of books) present these images to humans
decipher as words Captchas, as part of their procedure
validation normal. Then they return the results to the service
reCAPTCHA, which sends these results to the scanning of your
projects.