PHP : Using a variable to hold a table name, and using that variable in queries

$query = "INSERT INTO `" . $mysql_tb . "`
VALUES ('".$studno."','".$firstname."','".$lastname."')";

<?php
$mysql_tb = 'tb_registered_users';
$query = "SELECT `stud_no` FROM `{$mysql_tb}` WHERE `stud_no`='$studno'";
$query = "INSERT INTO `{$mysql_tb}` VALUES ('".$studno."','".$firstname."','".$lastname."')";