Before start you have to generate or collect recaptcha SITE_KEY and SECRET_KEY from admin console at https://www.google.com/recaptcha/

Three easy steps.

Step One

Add following scripts before </head> tag.

<!--Recaptcha script-->
<script src='https://www.google.com/recaptcha/api.js'></script>

<!-- Recaptcha field Required-->
<script>
window.onload = function() {
var recaptcha = document.forms["cvform"]["g-recaptcha-response"];
recaptcha.required = true;
recaptcha.oninvalid = function(e) {
// do something
alert("Please complete the captcha");
}
}
</script>

Step Two

Add following markup to the form where you want to show recaptcha.

<div class="g-recaptcha" data-sitekey="YOUR_RECAPCHA_SITE_KEY"> </div>

Step Three

Add following Recaptcha verification code snippet to the PHP script.

//reCAPTCHA validation
  if(isset($_POST['g-recaptcha-response']) && !empty($_POST['g-recaptcha-response']))
  {
        $secret = 'YOUR_RECAPTCHA_SECRET_KEY';
        $verifyResponse = file_get_contents('https://www.google.com/recaptcha/api/siteverify?secret='.$secret.'&response='.$_POST['g-recaptcha-response']);
        $responseData = json_decode($verifyResponse);

       if($responseData->success)
       {
           $succMsg = 'Your contact request have submitted successfully.';
       }
        else
       {
            $errMsg = 'Robot verification failed, please try again.';
       }
   }