PAD also runs on PHP compiled to WebAssembly, inside a browser tab, with no PAD server at all. php wasm/build.php bundles the engine into one file, and the page wasm/ loads PHP into the tab, writes the engine into its memory and renders the template you type with the JSON data beside it - the example below, on this server, is the page's sample:

PHP
fragments/pad_in_the_browser_1.php HTML
fragments/pad_in_the_browser_1.pad Result

PHP fragments/pad_in_the_browser_1.php	HTML fragments/pad_in_the_browser_1.pad	Result
`<?php $title = 'Fruit'; $items = [ [ 'name' => 'Apple', 'stock' => 3 ], [ 'name' => 'Pear', 'stock' => 0 ] ]; ?>`	`<h2>{$title}</h2> <ul> {items} <li>{$name}{if $stock eq 0} - sold out{/if}</li> {/items} </ul>`	Fruit Apple Pear - sold out

<?php

  $title = 'Fruit';
  $items = [ [ 'name' => 'Apple', 'stock' => 3 ], [ 'name' => 'Pear', 'stock' => 0 ] ];

?>

<h2>{$title}</h2>
<ul>
  {items}
    <li>{$name}{if $stock eq 0} - sold out{/if}</li>
  {/items}
</ul>

Fruit

Apple
Pear - sold out

PHP comes from the @php-wasm packages of WordPress Playground, loaded from jsDelivr: about 20 MB, once.
The template can only reach the browser tab it runs in, so there is no PHP function list to keep; there is no database either, and the sequence subsystem's large tables are left out of the bundle.
It is experimental: wasm/README.md says what was verified - the engine on the PHP 8.3, 8.4 and 8.5 WebAssembly builds, the page in Chrome - and what was not.

A whole application runs in the browser the same way, its SQLite database kept there between visits, opened from the disk or installed from an https host: Offline applications.