<?php

function registerusers(){
  $username =$_POST['username'];
  $password =$_POST['password'];
  $error = "Your Login Name or Password is invalid";
  echo "$username";
  $sql = "SELECT id FROM users WHERE username = '$username' and password = '$password'";
  $result = mysqli_query($conn,$sql);
  $row = mysqli_fetch_array($result,MYSQLI_ASSOC);
  $count = mysqli_num_rows($result);
  if($count == 1) {
  ?>
  <script type="text/javascript">
  $("div#ack").html("Successfully");
  </script>
  <?php
  }else {
  ?>
  <script type="text/javascript">
  $("div#ack").html("Invalid user and password");
  </script>
  <?php
  }
}
registerusers()

$("button#submit").click( function() {
    if( $("#username").val() == "" || $("#password").val() == "" ){
    $("div#ack").html("Please enter both username and password");
    }else{
    $.post( $("#myForm").attr("action"),
    $("#myForm :input").serializeArray(),
    function(data) {
    $("div#ack").html(data);
    });
    $("#myForm").submit( function() {
    return false;
    });
    }
});