与api和txt文件进行简单聊天

Question

I'm building a simple chat between two users and I would like to display a name of sender in the chat of recipient so it would appear on the screen of receiver like sender: sender's message to recipient.

HTML FORM:

<div id="lblMessages">
    </div>
    <form>
      <input name="txt-user-id" type="text" value="<?= $sUserId; ?>">
      <input name="txt-message" type="text">
      <button>send</button>
    </form>
  </div>

JS:

<script>

    let sUserId = '<?= $sUserId; ?>'

    $('form').submit( function(){
      $.ajax({
        method: "POST",
        url: "api-set-message.php",
        data: $('form').serialize(),
        cache: false
      }).
      done(function( sMessages ){
        console.log('done')
      }).
      fail(function(){

      })
      return false;
    })


    setInterval( function(){

      $.ajax({
        method: "GET",
        url: "api-get-messages.php?sUserId="+sUserId,
        cache: false
      }).
      done(function( sMessages ){
        $('#lblMessages').append('<div>'+sMessages+'</div>')
      }).
      fail(function(){

      })

    } , 1000 )

  </script>

API SET MESSAGE:

<?php
$sUserId = $_POST['txt-user-id'];
$sMessage = $_POST['txt-message'];
$sUserId = $sUserId == 'client' ? 'bank' : 'client';
file_put_contents( "to-$sUserId.txt", $sMessage );

API GET MESSAGE

<?php
$sUserId = $_GET['sUserId'];
$sMessages = file_get_contents("to-$sUserId.txt");
file_put_contents("to-$sUserId.txt", '');
echo $sMessages;