รบกวนสอบถามการใช้ if isset หลายครั้งหน่อยจ้าาาา มือใหม่ php

if (isset($_REQUEST['action']) && $_REQUEST['action'] == "SCAN") {

    if (isset($_POST['chk_machine']) && (isset($_POST['chk_time']) && (isset($_POST['barcode']) && $_POST['barcode']!==null))) {
      $sqlupdate ="update mdt_label set label_stamp=now(),label_in=now(),label_inby='".$_SESSION['login_user']."',
      label_machine=".$_POST['chk_machine'].",label_time=".$_POST['chk_time'].",label_status='3'
      where label_code='".$_POST['barcode']."'";
    

    if (isset($_POST['chk_machine']='0') && (isset($_POST['chk_time']) && (isset($_POST['barcode']) && $_POST['barcode']!==null))) {
      $sqlupdate ="update mdt_label set label_stamp=now(),label_in=now(),label_inby='".$_SESSION['login_user']."',
      label_time=".$_POST['chk_time'].",label_status='3'
      where label_code='".$_POST['barcode']."'";
      mysqli_query($db,$sqlupdate);
    }
  }