Can't get ReCAPTCHA to work (or even display!) - php

Question

I'm sure it's simple but I can't see it :)
I'm trying to add a reCAPTCHA into the code for an existing site with a little contact form.
I have the keys from Google (censored in the examples of course) but I can't even get it to display the CAPTCHA, let alone anything test if the filtering is working.
I've added the reCAPTCHA code into the page:
index.html:
form name="f1" method="post" action="mail2.php" onsubmit="return verify();">
<p><label>Your Name <span>(*)</span></label><input type="text" name="name" /></p>
<p><label>Your Email</label><input type="text" name="email" /></p>
<p><label>Your Phone No: <span>(*)</span></label><input type="text" name="phone" /></p>
<p><label>Other messages</label><textarea name="messages" rows="" cols=""></textarea> </p>
<?php
require_once('recaptchalib.php');
$publickey = "6LdrxxxxxxxxxxxxxxJr"; // you got this from the signup page
echo recaptcha_get_html($publickey);
?>
<p style="padding-right:36px; float:right;"><input name="" type="image" src="images/submit_btn.png" /></p>
</form>
And in the mail2.php
<?php
require_once('recaptchalib.php');
$privatekey = "6LxxxxxxxxxxxxxxxxxxqZYiH";
$resp = recaptcha_check_answer ($privatekey,
$_SERVER["REMOTE_ADDR"],
$_POST["recaptcha_challenge_field"],
$_POST["recaptcha_response_field"]);
if (!$resp->is_valid) {
// What happens when the CAPTCHA was entered incorrectly
die ("The reCAPTCHA wasn't entered correctly. Go back and try it again." .
"(reCAPTCHA said: " . $resp->error . ")");
} else {
// Your code here to handle a successful verification
require_once 'mailer/class.phpmailer.php';
$mail = new PHPMailer();
----followed by the standard PHPMailer content----
The index page just displays as standard with no CAPTCHA section displayed, trying to send an email results in failure with some PHP code displayed.
I'd really appreciate if someone with a mind larger than mine who eats & breathes reCAPTCHA could cast their eye, laugh & point at where I've gone wrong. :)
Looking to be educated rather than just a fix. My skills lie in content rather than coding I'm afraid.
(And how do I insert a code block in the editor? Surely don't have to indent each line by 4 spaces individually?)
Many Thanks.

Answer 1

I can help because I have it working farther than you, but I need help on this subject too.
To begin, you need this line of code in your html header (before the </head> tag:
<script src="https://www.google.com/recaptcha/api.js?fallback=true" async defer></script>
Then in the body of your html, and somewhere inside of your form (between your <form> ... </form> tags) you need the following code:
<div class="g-recaptcha" data-sitekey="....your site key here...."></div>
Then in your PHP processing file (the file that your form posts to and receives the form variables you need the following code:
$response_string = $_POST['g-recaptcha-response'];
$secret_key = "...put in your secret key here...";
$resp = file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=$secret_key&response=$response_string");
if($resp != '{ "success": true }')
{ $problem = "failedreCAPTCHA";
header("Location: http://www.yourwebsite.com/...pagetohandlefailedentry...php?problem=$problem");
die('Sorry, you entered the wrong value in the Human CAPTCHA challenge. Please try again.');
}
This code works great except for the "if($resp != '{ "success": true }')" part of it. Because the variable $resp get filled with from google is "{ "success": true }" which turns out to be a JSON object and I don't know how to test for "true" with PHP and a JSON object.
So... someone, please help.

Answer 2

This appears to be working for me, using Google's new "no Captcha reCaptcha". This largely builds off of Brook's answer, but includes the ip, parses the json, and tests for success.
<?php
$gSecret = "...put in your secret key here...";
$gResponse = $_POST['g-recaptcha-response'];
$gRemoteIp = $_SERVER['REMOTE_ADDR'];
$gResponse = file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=$gSecret&response=$gResponse&remoteip=$gRemoteIp");
$resp = json_decode($gResponse);
if (!$resp->success) {
// What happens when the CAPTCHA was entered incorrectly
die ("The reCAPTCHA wasn't entered correctly. Go back and try it again." .
"(reCAPTCHA said: " . $resp->error-codes . ")");
} else {
// Your code here to handle a successful verification
}
?>