<form method="post">
  <input type="text" id="username" placeholder="username" /><br>
  <input type="password" id="password" placeholder="password" /><br>
  <input type="button" id="btn" value="Login" />
</form>
<div id="feed_back"></div>
<script src="https://code.jquery.com/jquery-3.2.1.js"></script>
<script type="text/javascript">

$(document).ready(function(){
$('#btn').click(function(){
  var username = $('#username').val();
  var password = $('#password').val();
  $.ajax({
    type: 'POST',
    url: 'alma.php',
    data: {username:username, password:password},
    statusCode:{
      404: function() {
        $('#feed_back').html('A file not found');
      },
      000: function() {
        $('#feed_back').html('a file not found');
      }
    },
    succes: function(data){
      $('#feed_back').html(data);
    }
  });
});
});

</script>`
</body>`

<?php

  $username = $_POST['username'];
  $username = $_POST['password'];

  $username = strrev($username);
  echo $username.' '.$password;
?>