Pasted

<?php add_action('template_redirect', function () {
    $secrete_pass = 'allow-access';
    $maintenance_url = '/maintenance/';
    $protocol = 'http://';
    $user_is_admin = current_user_can('manage_options');
    
    if ($user_is_admin) return;
    
    if ( ($_GET['access'] ?? '') === $secrete_pass ) return;
    
    if ( ($_SERVER['HTTPS'] ?? '') === 'on' || ($_SERVER['HTTPS'] ?? 0) === 1 ||      ($_SERVER['HTTP_X_FORWARDED_PROTO'] ?? 'http') === 'https' )
    {
        $protocol = 'https://';
    }
    
    $path = wp_make_link_relative( $protocol . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'] );
    
    if ( $path !== $maintenance_url ) exit( wp_redirect( home_url( $maintenance_url ), 307 ));
}