Deleting table in a database

<?php
$server = 'localhost';
$username = 'root';
$password = '';
$db = 'TNW';
$conn = mysqli_connect($server, $username, $password, $db);
if($conn){
$query = "DROP TABLE `username`";
$query_run = mysqli_query($conn, $query);
if($query_run){
echo 'Successfully deleted a table.';
}else{
echo 'Deleting a table Error.';
}
}else{
echo 'Connecting to a database Error.';
}
?>