The following diagram represents the flow of request-response on the server side:

Request-response flow on the server side

The job of a web server is to route external requests to the correct application so that they can be processed. Once the application returns a response, the web server will send this response to the client. Let's take a close look at all the steps:

There are powerful web servers that support high loads of traffic, such as Apache or Nginx, which are fairly simple to install and manage. For the purpose of this book, though, we will use something even simpler: a PHP built-in server. The reason to use this is that you will not need extra package installations, configurations, and headaches as it comes with PHP. With just one command, you will have a web server running on your machine.

Finally, hands on! Let's try to create our very first web page using the built-in server. For this, create an index.php file inside your workspace directory—for example, Documents/workspace/index.php. The content of this file should be:

<?php
echo 'hello world';

Now, open your command line, go to your workspace directory, probably by running the cd Documents/workspace command, and run the following command:

$ php -S localhost:8000

The command line will prompt you with some information, the most important one being what is listening, which should be localhost:8000 as specified, and how to stop it, usually by pressing Ctrl + C. Do not close the command line as it will stop the web server too.

Now, let's open a browser and go to http://localhost:8000. You should see a hello world message on a white page. Yay, success! If you are interested, you can check your command line, and you will see log entries of each request you are sending via your browser.

So, how does it really work? Well, if you check again in the previous diagram, the php -S command started a web server—in our case, listening to port 8000 instead of 80. Also, PHP knows that the web application code will be on the same directory that you started the web server: your workspace. There are more specific options, but by default, PHP will try to execute the index.php file in your workspace.

Let's try to include our first project (index.html with its CSS and JS files) as part of the built-in server. To do this, you just need to open the command line and go to the directory in which these files are and start the web server with php -S localhost:8000. If you check localhost:8000 in your browser, you will see our two-link page, as is expected.

Let's now move our new index.php file to the same directory. You do not need to restart your web server; PHP will know about the changes automatically. Go to your browser and refresh the page. You should now see the hello world message instead of the links. What happened here?

If you do not change the default options, PHP will always try to find an index.php file in the directory in which you started the web server. If this is not found, PHP will try to find an index.html file. Previously, we only had the index.html file, so PHP failed to find index.php. Now that it can find its first option, index.php, it will load it.

If we want to see our index.html file from the browser, we can always specify it in the URL like http://localhost:8000/index.html. If the web server notices that you are trying to access a specific file, it will try to load it instead of the default options.

Finally, if we try to access a file that is not on our filesystem, the web server will return a response with status code 404—that is, not found. We can see this code if we open the Developer tools section of our browser and go to the Network section.