How to make a new session with customer id? PHP/Mysqli - php

Question

I have a login page which generates $_SESSION['username'] = $username.
I want to create new session $_SESSION['id'] which stores customer's id not username and pass it to another page.
I'm still learning so please make your answer understandable. Thanks
here's my login code
<?php
require('dbConfig.php');
session_start();
if(!empty($_SESSION["username"])){
header("LOCATION:index.php");
}
if (isset($_POST['username'])){
$username = stripslashes($_REQUEST['username']); // removes backslashes
$username = mysqli_real_escape_string($db,$username); //escapes special
characters in a string
$password = stripslashes($_REQUEST['password']);
$password = mysqli_real_escape_string($db,$password);
$query = "SELECT * FROM `members` WHERE username='$username' and password='".md5($password)."'";
$result = mysqli_query($db,$query) or die(mysql_error());
$rows = mysqli_num_rows($result);
if($rows==1){
$_SESSION['username'] = $username;
header("Location: index.php");
}else{
echo "<div class='form'><h3>Username/password is incorrect.</h3><br/>Click here to <a href='login.php'>Login</a></div>";
}
}else{
?>

Answer 1

Make sure session_start() is present is every page and add this in the session creation section of your code:
$_SESSION['id'] = $result['id'];
under
$_SESSION['username'] = $username;
I am asuming your "id" is called "id" in your table
You can use
echo "<pre>";
print_r($_SESSION);
echo "</pre>";
To check whats in your session

Answer 2

First of all: try to use mysqli_prepare to query user, by passing variable directly into SQL code, you open your system to SQL Injection.
$query = "SELECT * FROM `members` WHERE username=? and password=?"
$password = md5($password);
$statement = mysqli_prepare($db, $query);
mysqli_stmt_bind_param($statement, 'ss', $username, $password); //bind variables before execute sql command to prevent SQL injection.
mysqli_stmt_execute($statement);
$result = mysqli_stmt_get_result($statement);
$rows = mysqli_stmt_affected_rows($statement);
if($rows==1){
$user = mysqli_fetch_array($result, MYSQLI_ASSOC)
$_SESSION['username'] = $username;
$_SESSION['id'] = $user['id'];
header("Location: index.php");
}
Also, you must include session_start(); inside index.php in order to read SESSION variable on index.php page.