include("dbconnect.php"); //database connection

session_start();

$error = ""; //error msg variable.
if(isset($_POST["submit"])){
    if(empty($_POST["username"]) || empty($_POST["password"]))
    {
        $error = "username/password can't be empty.";
    }else
    {
        $username=$_POST['username'];
        $password=$_POST['password'];
        $username = stripslashes($username); //SQL Injection denial
        $password = stripslashes($password);
        $username = mysqli_real_escape_string($username, $koneksi);
        $password = mysqli_real_escape_string($password, $koneksi);
        $sql = "SELECT * FROM operator WHERE username='$username' and password='$password'";//username and password checking
        $result =mysqli_query($sql, $koneksi);
        $row = mysqli_fetch_array($result,MYSQLI_ASSOC);
    if(mysqli_num_rows($result) == 1){
        $_SESSION['username'] = $login_user; // create session
        header("location: home.php"); // redirection
    }else{
        $error = "wrong username or password.";
        }
    }
}

Warning: mysqli_real_escape_string() expects parameter 1 to be mysqli, string given in /opt/lampp/htdocs/libraryman/login.php on line 15

Warning: mysqli_real_escape_string() expects parameter 1 to be mysqli, string given in /opt/lampp/htdocs/libraryman/login.php on line 16

Warning: mysqli_query() expects parameter 1 to be mysqli, string given in /opt/lampp/htdocs/libraryman/login.php on line 18

Warning: mysqli_fetch_array() expects parameter 1 to be mysqli_result, null given in /opt/lampp/htdocs/libraryman/login.php on line 19

Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, null given in /opt/lampp/htdocs/libraryman/login.php on line 20