if ($_SERVER['REQUEST_METHOD'] == 'POST'){
        //two passwords are the same
        if($_POST['password'] == $_POST['confirmedpassword']) {

        $username = $mysqli->real_escape_string($_POST['username']);
        $password = md5($_POST['password']);

        $_SESSION['username'] = $username;
        $_SESSION['password'] = $password;

        $sql = "INSERT INTO members(username, password)"
            . "VALUES ('$username','$password')";

            //if querey is successful redirect to login.php
            if($mysqli->query($sql) === true)
                $_SESSION['message'] = 'Success';
                header("location: login.php");

    }
    else{
        $_SESSION['message'] = "User couldnt be added";

    }
}
else{
    $_SESSION['message'] = "Passwords dont match";
}

<?php
if ($_SERVER['REQUEST_METHOD'] == 'POST')
{
    //two passwords are the same
    if($_POST['password'] == $_POST['confirmedpassword'])
    {
        $username = $mysqli->real_escape_string($_POST['username']);

        // You might consider using salt when storing passwords like this
        $salt = 'aNiceDay';
        $password = md5(md5($_POST['password'].$salt).$salt);

        $_SESSION['username'] = $username;
        $_SESSION['password'] = $password;

        $sql = "SELECT `username` FROM members WHERE `username` = '".$username."'";
        $result = $mysqli->query($sql);

        if(mysqli_num_rows($result) > 0)
        {
            echo 'User exists.';
            // Do something.
        }
        else
        {
            $sql = "INSERT INTO members(username, password) VALUES ('".$username."','".$password."')";

            if($mysqli->query($sql) === true)
            {
                $_SESSION['message'] = 'Success';
                header("location: login.php");
                // Important to put exit() after header so other code
                // doesn't get executed.
                exit();
            }
            else
            {
                $_SESSION['message'] = "User couldn't be added";
                echo "User couldn't be added.";
            }
        }
    }
    else
    {
        $_SESSION['message'] = "Passwords dont match";
    }
}
?>

if ($_SERVER['REQUEST_METHOD'] == 'POST'){
        //two passwords are the same
        if($_POST['password'] == $_POST['confirmedpassword']) {

        $username = $mysqli->real_escape_string($_POST['username']);
        $password = md5($_POST['password']);

        $_SESSION['username'] = $username;
        $_SESSION['password'] = $password;

        //Check user
        $CheckUserIsExist = mysqli->query("SELECT uid FROM members WHERE username='$username'");
        if(mysqli_num_rows($CheckUserIsExist)==0 ){
        $sql = "INSERT INTO members(username, password)"
            . "VALUES ('$username','$password')";

            //if querey is successful redirect to login.php
            if($mysqli->query($sql) === true)
                $_SESSION['message'] = 'Success';
                header("location: login.php");

    }
   } else{
      echo 'This username is already in use. Plase use different username';
   }
    else{
        $_SESSION['message'] = "User couldnt be added";

    }
}
else{
    $_SESSION['message'] = "Passwords dont match";
}