local state for immediate stuff

runtime.php

<?php

function create_group($symbols, $prefix)
{
  return (object) [
    'tag' => 'group',
    'symbols' => $symbols,
    'prefix' => $prefix,
    'key' => 0,
    'used' => []
  ];
}

function current_group()
{
  global $stack;
  return $stack[count($stack) - 1];
}

function next_key()
{
  $group = current_group();
  return $group->prefix . $group->key++;
}

function clean_current_group()
{
  $group = current_group();
  foreach ($group->symbols as $key => $value) {
    if (!in_array($key, $group->used)) {
      unset($group->symbols->$key);
    }
  }
}

function &use_state($factory)
{
  $group = current_group();
  $key = next_key();
  $group->used[] = $key;
  if (!isset($group->symbols->$key)) {
    $group->symbols->$key = $factory();
  }
  return $group->symbols->$key;
}

function begin_group($key = null)
{
  global $stack;
  $group = current_group();
  if ($key === null) {
    $key = next_key();
  }
  $group->used[] = $key;
  if (!isset($group->symbols->$key)) {
    $group->symbols->$key = create_group(
      (object) [],
      $key . '-'
    );
  }
  $stack[] = &$group->symbols->$key;
}

function end_group()
{
  global $stack;
  clean_current_group();
  array_pop($stack);
}

function is_group($value)
{
  return is_object($value) && isset($value->tag) && $value->tag == 'group';
}

function sleep_group($group)
{
  $symbols = (object) [];
  foreach ($group->symbols as $key => $value) {
    if (is_group($value)) {
      $symbols->$key = sleep_group($value);
    } else {
      $symbols->$key = $value;
    }
  }
  return (object) [
    'tag' => 'group',
    'symbols' => $symbols
  ];
}

function wake_group($group, $prefix = '')
{
  $symbols = (object) [];
  foreach ($group->symbols as $key => $value) {
    if (is_group($value)) {
      $symbols->$key = wake_group($value, $prefix . $key . '-');
    } else {
      $symbols->$key = (array) $value;
    }
  }
  return create_group($symbols, $prefix);
}

runtime_test.php

<?php

require 'runtime.php';

function counter($action)
{
  extract(use_state(fn () => ['id' => uniqid(), 'count' => 0]), EXTR_REFS);
  if ($action == 'increment-' . $id) {
    $count++;
  }
?>
  <button name="action" value="increment-<?= $id ?>"><?= $count ?></button>
<?php
}

function render($action)
{
  extract(use_state(fn () => ['show' => false]), EXTR_REFS);
  if ($action == 'toggle') {
    $show = !$show;
  }
?>
  <button name="action" value="toggle">Toggle</button>
  <?php counter($action) ?>
  <?php begin_group() ?>
  <?php if ($show) : ?>
    <?php counter($action) ?>
  <?php endif ?>
  <?php end_group() ?>
  <?php counter($action) ?>
<?php
}

function tick()
{
  global $stack;
  $root = wake_group(json_decode($_POST['state'] ?? '{"symbols":{}}'));
  $stack = [$root];
?>
  <form method="post">
    <?php render($_POST['action'] ?? '') ?>
    <?php clean_current_group() ?>
    <input type="hidden" name="state" value="<?= htmlspecialchars(json_encode(sleep_group($root))) ?>">
  </form>
<?php
}

tick();