simple_login_session.php

<?php

// file connection.php
$con = mysqli_connect("localhost", "username", "password", "dbname");

// file login.php
require_once(__DIR__ . '/connection.php');

session_start();

// mysqli_real_escape_string prevent sql injection (security)
$user = mysqli_real_escape_string($con, $_POST['username']);
$pass = mysqli_real_escape_string($con, $_POST['password']);

//convert password to md5
$pass = md5($pass);

// check if the user id and password combination exist in database
$sql = mysqli_query($con, "SELECT * FROM table WHERE username = '$user' AND password = '$pass'") or die(mysqli_error($con));

//if match is equal to 1 there is a match
if (mysqli_num_rows($sql) === 1) {
    $user = mysqli_fetch_array($sql);

    //set session
    $_SESSION['authorized'] = true;
    $_SESSION['username'] = $user['username']; // username stored in session global variable

    // reload the page
    $_SESSION['success'] = 'Login Successful';
    header('Location: ./index.php');
    exit;
} else {
    // login failed save error to a session
    $_SESSION['error'] = 'Sorry, wrong username or password';
}