Add captcha to responsive bootstrap contact form - php

Question

I am fairly new to bootstrap. I have a contact form that is working properly. However, I would like to add a captcha to the form to help eliminate unwanted SPAM mail. How can I add a captcha to my existing form or any other "are you human" option to the form (i.e. would a simple checkbox work) and make it responsive as the current form is? Below is the code for my form:
<section id="contact">
<div class="container">
<div class="row">
<div class="col-lg-12 text-center">
<h2>Contact Me</h2>
<hr class="star-primary">
</div>
</div>
<div class="row">
<div class="col-lg-8 col-lg-offset-2">
<form name="sentMessage" id="contactForm" novalidate>
<div class="row control-group">
<div class="form-group col-xs-12 floating-label-form-group controls">
<label>Name</label>
<input type="text" class="form-control" placeholder="Name" id="name" required data-validation-required-message="Please enter your name.">
<p class="help-block text-danger"></p>
</div>
</div>
<div class="row control-group">
<div class="form-group col-xs-12 floating-label-form-group controls">
<label>Email Address</label>
<input type="email" class="form-control" placeholder="Email Address" id="email" required data-validation-required-message="Please enter your email address.">
<p class="help-block text-danger"></p>
</div>
</div>
<div class="row control-group">
<div class="form-group col-xs-12 floating-label-form-group controls">
<label>Phone Number</label>
<input type="tel" class="form-control" placeholder="Phone Number" id="phone" required data-validation-required-message="Please enter your phone number.">
<p class="help-block text-danger"></p>
</div>
</div>
<div class="row control-group">
<div class="form-group col-xs-12 floating-label-form-group controls">
<label>Message</label>
<textarea rows="5" class="form-control" placeholder="Message" id="message" required data-validation-required-message="Please enter a message."></textarea>
<p class="help-block text-danger"></p>
</div>
</div>
<br>
<div id="success"></div>
<div class="row">
<div class="form-group col-xs-12">
<button type="submit" class="btn btn-success btn-lg">Send</button>
</div>
</div>
</form>
</div>
</div>
</div>
</section>

Answer 1

Google ReCaptcha is a great tool for this and Google is also preparing a Captcha where no user input would be needed.
To implement a ReCaptcha, you will need to do following:
Create Google ReCaptcha API key
Paste the code into your HTML. Basically a div and some JS.
Validate the response when the form is submitted.
You can follow the step-by-step tutorial here: https://bootstrapious.com/p/bootstrap-recaptcha.