verifying recaptcha in php - php

Question

I have a simple .html contact form with recaptcha added to it and I have a correspond .php file set up to send the populated form fields information back to me via my email. After the form has been submitted, I redirect the user to my homepage. I'm trying to figure out what code is needed to add the recaptcha verification to the .php file and where, while still retaining the functionality I already have set up.
html code:
<script src='https://www.google.com/recaptcha/api.js' async defer>
</script>
<form method="post" action="mail.php">
<input id="name" name="name" placeholder="Name" required />
<input id="email" name="email" placeholder="Email" type="email" required />
<textarea id="message" name="message" placeholder="Question/Comment" required></textarea>
<div class="g-recaptcha" data-sitekey="MY SITE KEY"></div>
<input class="btn-success formBtn" name="submit" type="submit" />
<input class="formBtn" type="reset" />
</form>
php code:
<?php
$name = $_POST['name'];
$email = $_POST['email'];
$message = $_POST['message'];
$formcontent="From: $name \n Message: $message";
$recipient = "MY EMAIL";
$subject = "Contact Form";
$mailheader = "From: $email \r\n";
mail($recipient, $subject, $formcontent, $mailheader) or
die("Error!");
header('Location: /');
?>

Answer 1

your html code seems good, you can try the following php code to see if it works with your form. also make sure you are using the v2 reCaptcha key.
php code
<?php
if(isset($_REQUEST['submit'])){
$captcha = $_REQUEST['g-recaptcha-response'];
$handle = curl_init('https://www.google.com/recaptcha/api/siteverify');
curl_setopt($handle, CURLOPT_POST, true);
curl_setopt($handle, CURLOPT_POSTFIELDS, "secret=YOUR_SECRET_KEY&response=$captcha");
curl_setopt($handle, CURLOPT_RETURNTRANSFER, true);
$response = curl_exec($handle);
$explodedArr = explode(",",$response);
$doubleExplodedArr = explode(":",$explodedArr[0]);
$captchaConfirmation = end($doubleExplodedArr);
if(trim($captchaConfirmation) == "true") {
$to ='example#email.com';
$subject = 'Form';
$headers = "From: " . $_POST['email'] . "\r\n";
$headers .= "MIME-Version: 1.0\r\n";
$headers .= "Content-Type: text/html; charset=ISO-8859-1\r\n";
$message = '';
$message .= "<strong>First Name:</strong> " .$_POST['name'] ;
$message .= "<strong>Email:</strong> " . $_POST['email'];
$message .= "<strong>Message:</strong> " . $_POST['message'] ;
$send = mail($to, $subject, $message, $headers);
if($send) {
echo "Message Sent. Thank You!";
} else {
echo "<script> alert('Message Not Sent. Please try again.";
}
} else {
echo "Captcha entry was wrong. Please try again.";
}
}
?>