data not registered(inserted) in sql using php

Question

I used same version of xampp though.....we are in a project and the code easily allow data to be inserted when I used my friend laptop(same xampp,same code) but when I register(try to register) ,it doesn't show any errors and the data is not registered... the code even display error message such as username required!!! etc when empty.
my code

$username = "";
$batch = "";
$errors = array();
/*if( ! empty( $_POST )) {
print_r($_POST);}*/
//connect to the database
$db = mysqli_connect('localhost', 'root', '', 'registration');
/*if($db->connect_error){
echo'sorry';
}*/
//if register button is clicked
//link with the register
if(isset($_POST['register'])){
$username = $_POST['username'];
$batch = $_POST['batch'];
$password_1 = $_POST['password_1'];
$password_2 = $_POST['password_2'];
$hash = md5(rand(0,1000));

if(empty($username)){
array_push($errors,"Username is required!!!");
}
if(empty($batch)){
array_push($errors,"Batch is required!!!");
}
if(empty($password_1)){
array_push($errors,"Password is required!!!");
}
if($password_1 != $password_2)
{
array_push($errors,"password do not match!!!");
}
//if no error save user to database
if (count($errors)==0)
{
$password = md5($password_1);//encrypt password before storing in database(security)

$sql = "INSERT INTO users (username, batch, password_1, password_2,hash) "

. "VALUES ('$username','$batch','$password_1','$password_2', '$hash')";

$_SESSION['username'] = $username;
$_SESSION['success'] = "you are now registered ^_^";
header('location: index.php'); //redirect to home page

}
}

Answer 1

you never pass the command in $sql to your database. that's probably why it's not working.
also for security reasons you might want to escape any passed string (username etc) with mysqli_real_escape_string(). otherwise it would be easy to use sql injections and mess with your database ;)