ReCaptcha Library for CodeIgniter (but cant be used for any PHP projet). I didn't test everything

recaptcha.php

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed'); 
/*
 * This is a PHP library that handles calling reCAPTCHA.
 *    - Documentation and latest version
 *          http://recaptcha.net/plugins/php/
 *    - Get a reCAPTCHA API Key
 *          https://www.google.com/recaptcha/admin/create
 *    - Discussion group
 *          http://groups.google.com/group/recaptcha
 *
 * Copyright (c) 2007 reCAPTCHA -- http://recaptcha.net
 * AUTHORS:
 *   Mike Crawford
 *   Ben Maurer
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

/**
 * A ReCaptchaResponse is returned from check_answer()
*/
class ReCaptchaResponse {
	public $is_valid;
	public $error;
}

class Recaptcha {
	/**
	 * The reCAPTCHA server URL's
	*/
	private $apiServer = 'http://www.google.com/recaptcha/api';
	private $apiServerSecure = 'https://www.google.com/recaptcha/api';
	private $verifyServer = 'www.google.com';

	private $publicKey = 'PUBLIC';
	private $privateKey = 'PRIVATE';


	/**
	 * Encodes the given data into a query string format
	 * @param $data - array of string elements to be encoded
	 * @return string - encoded request
	*/
	private function qsencode(){
		$req = '';

		foreach ($data as $key => $value)
			$req .= $key . '=' . urlencode( stripslashes($value) ) . '&';
		
		$req = substr($req, 0, strlen($req) - 1);

		return $req;
	}

	/**
	 * Submits an HTTP POST to a reCAPTCHA server
	 * @param string $host
	 * @param string $path
	 * @param array $data
	 * @param int port
	 * @return array response
	 */
	private function http_post($host, $path, $data, $port = 80) {

		$req = $this->qsencode ($data);

		$http_request  = "POST $path HTTP/1.0\r\n";
		$http_request .= "Host: $host\r\n";
		$http_request .= "Content-Type: application/x-www-form-urlencoded;\r\n";
		$http_request .= "Content-Length: " . strlen($req) . "\r\n";
		$http_request .= "User-Agent: reCAPTCHA/PHP\r\n";
		$http_request .= "\r\n";
		$http_request .= $req;

		$response = '';
		if( false == ( $fs = @fsockopen($host, $port, $errno, $errstr, 10) ) ) {
			die ('Could not open socket');
		}

		fwrite($fs, $http_request);

		while ( !feof($fs) )
			$response .= fgets($fs, 1160); // One TCP-IP packet
		
		fclose($fs);
		$response = explode("\r\n\r\n", $response, 2);

		return $response;
	}

	/**
	 * Gets the challenge HTML (javascript and non-javascript version).
	 * This is called from the browser, and the resulting reCAPTCHA HTML widget
	 * is embedded within the HTML form it was called from.
	 * @param string $pubkey A public key for reCAPTCHA
	 * @param string $error The error given by reCAPTCHA (optional, default is null)
	 * @param boolean $use_ssl Should the request be made over ssl? (optional, default is false)
	 * @return string - The HTML to be embedded in the user's form.
	*/
	public function get_html ($error = null, $use_ssl = false) {
		
		if ($use_ssl) {
			$server = $this->apiServerSecure;
		}
		else {
			$server = $this->apiServer;
		}

		$errorpart = '';
		if ($error) {
			$errorpart = "&amp;error=" . $error;
		}

		return '
			<script type="text/javascript" src="'. $server . '/challenge?k=' . $this->publicKey . $errorpart . '"></script>
			<noscript>
				<iframe src="'. $server . '/noscript?k=' . $this->publicKey . $errorpart . '" height="300" width="500" frameborder="0"></iframe><br/>
				<textarea name="recaptcha_challenge_field" rows="3" cols="40"></textarea>
				<input type="hidden" name="recaptcha_response_field" value="manual_challenge"/>
			</noscript>
		';
	}

	/**
	 * Calls an HTTP POST function to verify if the user's guess was correct
	 * @param string $privkey
	 * @param string $remoteip
	 * @param string $challenge
	 * @param string $response
	 * @param array $extra_params an array of extra variables to post to the server
	 * @return ReCaptchaResponse
	*/
	public function check_answer ($remoteip, $challenge, $response, $extra_params = array()) {

		if ($remoteip == null || $remoteip == '') {
			die ('For security reasons, you must pass the remote ip to reCAPTCHA');
		}
		
		//discard spam submissions
		if ($challenge == null || strlen($challenge) == 0 || $response == null || strlen($response) == 0) {
			$recaptcha_response = new ReCaptchaResponse();
			$recaptcha_response->is_valid = false;
			$recaptcha_response->error = 'incorrect-captcha-sol';
			return $recaptcha_response;
		}
		
		$response = $this->http_post ($this->verifyServer, '/recaptcha/api/verify', array (
				'privatekey' => $this->privateKey,
				'remoteip' => $remoteip,
				'challenge' => $challenge,
				'response' => $response
			) + $extra_params
		);

		$answers = explode ("\n", $response [1]);
		$recaptcha_response = new ReCaptchaResponse();

		if (trim ($answers [0]) == 'true') {
			$recaptcha_response->is_valid = true;
		}
		else {
			$recaptcha_response->is_valid = false;
			$recaptcha_response->error = $answers [1];
		}
		return $recaptcha_response;
	}

	/**
	 * gets a URL where the user can sign up for reCAPTCHA. If your application
	 * has a configuration page where you enter a key, you should provide a link
	 * using this function.
	 * @param string $domain The domain where the page is hosted
	 * @param string $appname The name of your application
	*/
	public function get_signup_url ($domain = null, $appname = null) {
		return "https://www.google.com/recaptcha/admin/create?" .  $this->qsencode (array ('domains' => $domain, 'app' => $appname));
	}

	private function aes_pad($val) {
		$block_size = 16;
		$numpad = $block_size - (strlen ($val) % $block_size);
		return str_pad($val, strlen ($val) + $numpad, chr($numpad));
	}

	/* Mailhide related code */
	private function aes_encrypt($val, $ky) {
		if (! function_exists ('mcrypt_encrypt')) {
			die ('To use reCAPTCHA Mailhide, you need to have the mcrypt php module installed.');
		}
		$mode = MCRYPT_MODE_CBC;   
		$enc = MCRYPT_RIJNDAEL_128;
		$val = aes_pad($val);
		return mcrypt_encrypt($enc, $ky, $val, $mode, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0");
	}

	private function mailhide_urlbase64 ($x) {
		return strtr(base64_encode ($x), '+/', '-_');
	}

	/* gets the reCAPTCHA Mailhide url for a given email, public key and private key */
	public function mailhide_url($pubkey, $privkey, $email) {

		$ky = pack('H*', $this->privateKey);
		$cryptmail = aes_encrypt ($email, $ky);
		
		return 'http://www.google.com/recaptcha/mailhide/d?k=' . $this->publicKey . '&c=' . mailhide_urlbase64 ($cryptmail);
	}

	/**
	 * gets the parts of the email to expose to the user.
	 * eg, given johndoe@example,com return ["john", "example.com"].
	 * the email is then displayed as john...@example.com
	*/
	private function mailhide_email_parts ($email) {
		$arr = preg_split("/@/", $email );

		if (strlen ($arr[0]) <= 4) {
			$arr[0] = substr ($arr[0], 0, 1);
		} else if (strlen ($arr[0]) <= 6) {
			$arr[0] = substr ($arr[0], 0, 3);
		} else {
			$arr[0] = substr ($arr[0], 0, 4);
		}
		return $arr;
	}

	/**
	 * Gets html to display an email address given a public an private key.
	 * to get a key, go to:
	 *
	 * http://www.google.com/recaptcha/mailhide/apikey
	*/
	public function mailhide_html($email) {
		$emailparts = mailhide_email_parts ($email);
		$url = mailhide_url ($email);
		
		return htmlentities($emailparts[0]) . "<a href='" . htmlentities ($url) .
			"' onclick=\"window.open('" . htmlentities ($url) . "', '', 'toolbar=0,scrollbars=0,location=0,statusbar=0,menubar=0,resizable=0,width=500,height=300'); return false;\" title=\"Reveal this e-mail address\">...</a>@" . htmlentities ($emailparts [1]);

	}
}