I want to get this piece of Login code to direct a user to a back_office.html page but I cannot get it to perform as required!

PHP Code:
<?php
session_start
(); 
$logins = array(
'u1' => 'p1',
'u2' => 'p2',
'u3' => 'p3',
'u4' => 'p4',
'u5' => 'p5'
);

$user strtolower($_POST['user']);
$pass $_POST['pass']; 

if (isset(
$logins[$user])&&$logins[$user]==$pass) {
    
$_SESSION['username'] = $user;


if (!isset(
$_SESSION['username'])) {
    exit(
'Access denied.'); 
}
?>
Any direction to get working greatly appreciated.