Submitted chat not posting and retrieving chat not working

Question

I using the below php chat script to create chat section between two users on my web app. I am having a problem with the Ajax posting. When a user submits a chat it doesn't post or show in the chat window. I tried to inspect the error and this is the error message

Failed to load resource: the server responded with a status of 404 (Not Found)

The same error message is shown for submit.php and refresh.php.

Here's my code:

JS

//CHAT FUNCTION

var lastTimeID = 0;

$(document).ready(function() {
  $('#btnSend').click( function() {
    sendChatText();
    $('#chatInput').val("");
  });
  startChat();
});

function startChat(){
  setInterval( function() { getChatText(); }, 2000);
}

function getChatText() {
  $.ajax({
    type: "GET",
    url: "refresh.php?lastTimeID=" + lastTimeID
  }).done( function( data )
  {
    var jsonData = JSON.parse(data);
    var jsonLength = jsonData.results.length;
    var html = "";
    for (var i = 0; i < jsonLength; i++) {
      var result = jsonData.results[i];
      html += '<div style="color:#' + result.color + '">(' + result.chattime + ') <b>' + result.usrname +'</b>: ' + result.chattext + '</div>';
      lastTimeID = result.id;
    }
    $('#view_ajax').append(html);
  });
}

function sendChatText(){
  var chatInput = $('#chatInput').val();
  if(chatInput != ""){
    $.ajax({
      type: "GET",
      url: "submit.php?chattext=" + encodeURIComponent( chatInput )
    });
  }
}

chatClass.php

<?PHP
 class chatClass
  {
    public static function getRestChatLines($id)
    {
      $arr = array();
      $jsonData = '{"results":[';
      $statement = $db->prepare( "SELECT id, usrname, color, chattext, chattime FROM chat WHERE id > ? and chattime >= DATE_SUB(NOW(), INTERVAL 1 HOUR)");
      $statement->bind_param( 'i', $id);
      $statement->execute();
      $statement->bind_result( $id, $usrname, $color, $chattext, $chattime);
      $line = new stdClass;
      while ($statement->fetch()) {
        $line->id = $id;
        $line->usrname = $usrname;
        $line->color = $color;
        $line->chattext = $chattext;
        $line->chattime = date('H:i:s', strtotime($chattime));
        $arr[] = json_encode($line);
      }
      $statement->close();
      $jsonData .= implode(",", $arr);
      $jsonData .= ']}';
      return $jsonData;
    }

    public static function setChatLines( $chattext, $usrname, $color) {
      $statement = $db->prepare( "INSERT INTO chat( usrname, color, chattext) VALUES(?, ?, ?)");
      $statement->bind_param( 'sss', $usrname, $color, $chattext);
      $statement->execute();
      $statement->close();
    }
  }
  ?>

submit.php

<?php
  require_once( "chatClass.php" );
  $chattext = htmlspecialchars( $_GET['chattext'] );
  chatClass::setChatLines( $chattext, $_SESSION['usrname'], $_SESSION['color']);
?>

refresh.php

<?php
  require_once( "chatClass.php" );
  $id = intval( $_GET[ 'lastTimeID' ] );
  $jsonData = chatClass::getRestChatLines( $id );
  print $jsonData;
?>