Add reCAPTCHA V2 - php

Question

it is already 1 week that I am trying to add a reCAPTCHA V2 module on my static website. (I bought the html5 template since I do not have any experience with web-dev).
That's my html part:
<form action="include/sendmail.php" method="post">
<div class="field">
<label class="required" for="sc_contact_form_username">Name</label>
<input type="text" name="username" id="sc_contact_form_username" />
</div>
<div class="field">
<label class="required" for="sc_contact_form_email">Email</label>
<input type="text" name="email" id="sc_contact_form_email" />
</div>
<div class="field message">
<label class="required" for="sc_contact_form_message">Your Message</label>
<textarea name="message" id="sc_contact_form_message"></textarea>
</div>
<div class="g-recaptcha" data-theme="dark" data-sitekey="MYKEY"></div>
<div class="button"> <a class="enter" href="#"><span>Submit</span></a> </div>
</form>
<div class="result sc_infobox"></div>
So far so good...the difficult part for me it's the PHP, how do I modify the current PHP to add the validation and make it more secure?
<?php
global $_REQUEST;
$response = array('error'=>'');
$user_name = substr($_REQUEST['user_name'], 0, 20);
$user_email = substr($_REQUEST['user_email'], 0, 40);
$user_msg = $_REQUEST['user_msg'];
$contact_email = 'your_mail#mail.com';
if (trim($contact_email)!='') {
$subj = 'Message from ShiftCV HTML';
$msg = "Name: $user_name
E-mail: $user_email
Message: $user_msg";
$head = "Content-Type: text/plain; charset=\"utf-8\"\n"
. "X-Mailer: PHP/" . phpversion() . "\n"
. "Reply-To: $user_email\n"
. "To: $contact_email\n"
. "From: $user_email\n";
if (!#mail($contact_email, $subj, $msg, $head)) {
$response['error'] = 'Error send message!';
}
} else
$response['error'] = 'Error send message!';
echo json_encode($response);
die();
?>

Answer 1

I would suggest you have a look at this composser that you can add to your application to make the required calls and validate the reCaptcha reponse: https://github.com/google/recaptcha