พี่ครับ ถ้า จะให้ echo json_encode return ค่าออกมาบอกว่าข้อมูลนี้ไม่มี ควรทำยังไงอะครับ

<?php

$con=mysqli_connect("mysql.hostinger.in.th","am","boi","u677770246_ae2de");

// Check connection
if (mysqli_connect_errno($con))
{
  echo "Failed to connect to MySQL: " . 	mysqli_connect_error();
}

$setting_id=$_POST["setting_id"];
$student_id=$_POST["STUDENT_ID"];
$result = mysqli_query($con,"SELECT settingproject.setting_id,settingproject.week,student.student_id,term, name_s,student.lastname FROM settingproject, student
WHERE student.setting_id = settingproject.setting_id AND settingproject.setting_id = $setting_id AND student.student_id= $student_id");

$arr=array();

while($row = mysqli_fetch_object($result))      	
{
$arr[] = $row;
}


echo json_encode($arr);

mysqli_close($con);
?>