Validation Formulaire HTML en AJAX , MySQL et PHP

<div class="col-md-8" id="comments">
      <div class="conp-right wow fadeInRight" data-wow-delay=".1s">
       <h3>Contact Form</h3>
       <form method="POST" class="contact-form" onSubmit="alert('Thank you for your feedback!');">
        <div class="row">
         <div class="col-md-4">
          <div class="panel-body"><input type="text" id="name" name="name"  placeholder="Your Name"/></div>
         </div>
         <div class="col-md-4">
          <div class="panel-body"><input type="text" id="email" name="email" placeholder="Your Email"/></div>
         </div>
         <div class="col-md-4">
          <div class="panel-body"><input type="text" id="subject" name="subject" placeholder="Subjects"/></div>
         </div>
         <div class="col-md-12">
          <div class="panel-body"><textarea id="comments" name="comments" rows="5" placeholder="Your Message"></textarea></div>
         </div>
         <div class="col-md-12">
          <div class="suc4 conp-btn text-center"><button type="submit" name="submit">SEND EMAIL</button></div>
         </div>
        </div>
       </form>
      </div>
     </div>