how to give error user already exists in php code example

Example: how to check user already exists in php

$sql = "SELECT username FROM table_name WHERE username='{$username}'";
$result = mysqli_query($con,$sql) or die("Query unsuccessful") ;
      if (mysqli_num_rows($result) > 0) {
        echo "Username is already exist";
      } else {
             ...............   
      }