Validation for username not working - php

Question

I have written this code , its working to add users but for duplicate user it again saved the value os same user name. i wanted to give popup message if username already taken. i m beginner please help.
<?php
ob_start();
include("db.php");
if(isset($_POST['send'])!="") {
$username = mysqli_real_escape_string($con, $_POST['username']);
$usermail = mysqli_real_escape_string($con, $_POST['usermail']);
$usermobile = mysqli_real_escape_string($con, $_POST['usermobile']);
$bool = true;
$con = mysqli_connect("localhost","root","","demo");
$result = mysqli_query("SELECT `username` FROM `sample` WHERE username = '$username'");
if(mysqli_num_rows($result)>0)
{
Print '<script>alert("Username has been taken!");</script>';
}
if ($bool) {
$inssql = "insert into sample set
username = '" . $username . "',
emailid = '" . $usermail . "',
mobileno = '" . $usermobile . "',
created = now()";
$update = mysqli_query($con, $inssql);
}
}

Answer 1

Make sure you finish the script or turn off your flag before making the insert:
if(mysqli_num_rows($result)>0)
{
Print '<script>alert("Username has been taken!");</script>';
die('username already taken');
//$bool = FALSE;
}
If you still having duplicate entries, debug what is the result of $username and compare it with the value in the database.