อยากจะให้แสดง alert ว่า username นี้ถูกใช้ไปแล้วครับ ขอคำแนะนำครับ

     <?PHP include_once 'include/condb.php'; ?> 

     <?PHP 
          

       
       
try{
   $dbh->beginTransaction();  
    
               //if  แบบย่อ
               //$x == 1 ? "OK" : "NO";
  
               // input form
               $user_id= $_POST['user_id'];
               $loginname=!empty($_POST['loginname'])?$_POST['loginname']:null;
	
               if(isset($_POST['password'])){$password=$_POST['password'];}
               $name=!empty($_POST['name'])?$_POST['name']:null;
			   $pid=!empty($_POST['pid'])?$_POST['pid']:null;
			   $tel=!empty($_POST['tel'])?$_POST['tel']:null;
	           $email=!empty($_POST['email'])?$_POST['email']:null;
               $priority_id=!empty($_POST['priority_id'])?$_POST['priority_id']:null;
               

              if($user_id==0){ // Add
                $sth=$dbh->prepare('insert into user (loginname,password,name,pid,tel,email,priority_id) values (?,md5(?),?,?,?,?,?)');
                $result=$sth->execute(array($loginname,$password,$name,$pid,$tel,$email,$priority_id));
              
              } else { // Edit
                $sth=$dbh->prepare('update user set loginname=?,password=md5(?),name=?,pid=?,tel=?,email=?,priority_id=? where user_id=?');
                $result=$sth->execute(array($loginname,$password,$name,$pid,$tel,$email,$priority_id,$user_id)); 
              }
              
	
              $dbh->commit();   // complete  
                if($result>0){
                     $json=array('status'=>'Yes');
                }else{
                     $json=array('status'=>'No');
                }

               echo json_encode($json);
    
        
    }catch(exception $e){
         $dbh->rollback();
         echo "Error :".$e->getMessage();
     }   
     
  require_once 'include/globals_end.php' ;