$(document).ready(function(){
  $(".show").click(function (){
    $(".web, .about, .tel, .email").hide();
    $("." + $(this).attr("show-class")).fadeIn("fast");
  });
});

<li><a href="#" class="show" show-class="email">E-mail</a></li>

<div class="email">

  <form method="post" action="">
  <input name="name" placeholder="Name (required)">
  <input name="email" type="email" placeholder="E-mail (required)">
  <textarea name="message" placeholder="Message"></textarea>
  <input name="submit" type="submit" value="Submit">
  </form>

  <?php

  $name = $_POST['name'];
  $email = $_POST['email'];
  $message = $_POST['message'];
  $to = 'example@example.com';
  $subject = 'Hi! From Example.com';

  $body = "From: $namen E-Mail: $emailn Message:n $message";

  if ($_POST['submit']) {
  if ($name != '' && $email != '') {
  if (mail ($to, $subject, $body)) {
  echo '<p>Your message has been sent.</p>';
  } else {
  echo '<p>Something went wrong, go back and try again.</p>';
  }
  } else {
  echo '<p>Please fill in required information.</p>';
  }
  }
  ?>

</div>