I bet you already used 3v4l.org or similar websites to run PHP code to find out whether array_merge is slow or not (it's slow). If you take a look at how these website works they usually use a PHP server behind the doors and transfer data between the browser and the server.

The problem is that running PHP on the server just for testing purposes can quickly become expensive in terms of hosting... How about running PHP directly in the browser?

In this talk, I'll introduce you to WebAssembly, a technology that enables programs to be compiled to run in browsers. Next, we'll discover how to compile PHP into WebAssembly. And finally, we'll see how to run Symfony or any other PHP project directly in the browser, without the need for a (PHP) server.