Google reCAPTCHA is becoming popular since the day of its announcement. The performance of this product is not been evaluated yet because of its age.

I have covered basic tutorial about how to code custom script with Google reCAPTCHA.

functions.php

/*Add Google captcha field to Comment form*/

add_filter('comment_form','add_google_captcha');

function add_google_captcha(){
echo '<div class="g-recaptcha" data-sitekey= "=== Your site key === "></div>';
}

/*End of Google captcha*/

single.php

<script src='https://www.google.com/recaptcha/api.js'></script>
<script type="text/javascript">
jQuery("#submit").click(function(e){
var data_2;
jQuery.ajax({
type: "POST",
url: "http://yourblog.com/wp-content/themes/yourtheme/google_captcha.php",
data: jQuery('#commentform').serialize(),
async:false,
success: function(data) {
if(data.nocaptcha==="true") {
data_2=1;
} else if(data.spam==="true") {
data_2=1;
} else {
data_2=0;
}
}
});
if(data_2!=0) {
e.preventDefault();
if(data_2==1) {
alert("Please check the captcha");
} else {
alert("Please Don't spam");
}
} else {
jQuery("#commentform").submit
}
});
</script>

google_captcha.php

<?php
$data;
header('Content-Type: application/json');
error_reporting(E_ALL ^ E_NOTICE);
if(isset($_POST['g-recaptcha-response'])) {
$captcha=$_POST['g-recaptcha-response'];
}
if(!$captcha){
$data=array('nocaptcha' => 'true');
echo json_encode($data);
exit;
}
// calling google recaptcha api.
$response=file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=Your secret key&response=".$captcha."&remoteip=".$_SERVER['REMOTE_ADDR']);
// validating result.
if($response.success==false) {
$data=array('spam' => 'true');
echo json_encode($data);
} else {
$data=array('spam' => 'false');
echo json_encode($data);
}
?>

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