toc.php

<?php

$data = array_map(function ($elem) {
    return ['elem' => $elem, 'nested' => []];
}, [1, 2, 2, 3, 4, 1, 2, 1, 1, 2, 2]);

function group($data) {
    $subs = false;
    $data = array_chunk($data, 2);
    $results = [];
    foreach ($data as $chunk) {
        if (count($chunk) == 1) {
            $results[] = $chunk[0];
        } else {
            $nested = nest($chunk);
            $subs = $subs || count($nested) == 1;
            $results = array_merge($results, $nested);
        }
    }

    return $subs ? group($results) : $results;
}

function nest($chunk) {
    if (count($chunk[0]['nested']) && array_search($chunk[1]['elem'], array_column($chunk[0]['nested'], 'elem')) !== false) {
        $chunk[0]['nested'][] = $chunk[1];
        return [$chunk[0]];
    }

    if ($chunk[1]['elem'] <= $chunk[0]['elem']) {
        return $chunk;
    }

    if (count($chunk[0]['nested']) == 0) {
        $chunk[0]['nested'][] = $chunk[1];
        return [$chunk[0]];
    }

    $nested = end($chunk[0]['nested']);
    while (is_array($nested)) {
        $result = nest([$nested, $chunk[1]]);
        if (count($result) == 1) {
            return [$chunk[0]];
        }
        $nested = end($nested['nested']);
    }

    return $chunk;
}

var_dump(group($data));