I have made some adjustments to my code and now is working , i just wonder if this is a good and safe way or i have to change or add something to the code.
<?php 
session_start();
$db = mysqli_connect("localhost", "root", "", "store");
if (isset($_POST['register_btn']))  {
    $username= mysqli_real_escape_string($db, $_POST['username']);
    $email = mysqli_real_escape_string($db, $_POST['email']);
    $pas= mysqli_real_escape_string($db, $_POST['pas']);
    $stmt = $db->prepare("INSERT INTO users (username,email,pas) 
                          VALUES(?,?,?)");
    $stmt->bind_param("sss", $username, $email, $pas);
    $stmt->execute();       
    mysqli_query($db, $stmt);
    header("location: login.php");
}
?>
 
    