simple login example

<?php

global $db = null; // db connection

$user_id = "";
$password = "";

/**
 * This function will fetch a specific user fro mthe database;
 *
 * @param int $user_id The user id;
 * @return Object it's fetched object. null will be returned if anything goes wrong.
 **/
function get_user($user_id) {
    // validating input
    // must not be empty or not a number nor a negative number
    if (empty($user_id) || !is_numeric($user_id) || $user_id <= 0 ) {
        return null;
    }

    // query to the db
    // please adjust to your case
    $user = $db->query("select * from users where id =  " . $user_id . ";");

    // returning values
    return ($user) ? $user : null;
}

function has_privilege($user_id, $role) {
    $query = $db->query("select * from roles where user_id = " . $user_id . ";");

    return ($role == $query['role']);
}

$user = get_user($user_id);

// loging in
// this presumes you're using PHP built-in validation functions see manual
if (password_verify($password, $user['password'])) {
    // checking if it has required privileges...
    if (has_privilege($user_id, "admin")) {
        // grant access
        header("location: ... "); // your page
    } else {
        // error. redirect to login
    }
} else {
    // error. redirect to login
}