<?php 

$states = ['Přijatá', 'Nová', 'K zaplacení', 'Zaplaceno', 'Expedováno', 'Doručeno'];
$currentState = $_GET['state'] ?? 'K zaplacení';

$currentIndex = array_search($currentState, $states);
$prevAll = array_slice($states, 0, $currentIndex);
$prev = end($prevAll) ?: null;
$nextAll = array_slice($states, $currentIndex + 1);
$next = $nextAll[0] ?? null;

$meta = [
    'prev' => $prev,
    'prevAll' => $prevAll,
    'next' => $next,
    'nextAll' => $nextAll,
    'currentStateNumber' => $currentIndex + 1,
    'totalStates' => count($states),
];

echo '<pre>';
print_r($meta);