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Got asked this interesting interview question today.

Explain in detail the process by which a client machine requests a file (say file.php) from the server, and then receives that desired file along with its necessary JS/CSS/images/video files and they appear on the client's browser screen.

Here is what I do know and what I did say:

So a request is sent, then the server sees that the file.php file is being requested, and because it has a .php extension, it first uses the PHP engine to parse any PHP code inside the file, and then once it is done, it outputs back to the client machine the resulting file.php file (as a response). The browser then takes that response and parses the HTML and necessary JS and CSS code, then displays it to the browser.

My answer is pretty basic and not as detailed as it should be. I thought about my response and came up with new questions:

  1. What, literally, is a "request"? Is it basically just the textual header file that gets sent to the server?

  2. What about a "response"? Is the response itself the parsed file.php file that gets sent back to the client machine?

  3. What if the file.php file contains a reference to a script.js file and a style.css file? At which stage do those files get served back to the client machine? Do they come in as separate headers or what?

  4. Above in my answer, I'm not sure if I was correct when I said "...because it has a .php extension, it first uses the PHP engine to parse any PHP code inside the file." Is that really the reason why the server parses the code inside the file, or does the server scan ALL kinds of files by default to check for any PHP code they might contain?

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First of all, I think your answer was quite good. It definitely describes the basic process you were asked about.

1) What, literally, is a "request"? Is it basically just the textual header file that gets sent to the server?

Yes, an HTTP request is a text message to a server including, mostly: the requested path, any parameters to that path, client info (user agent, session, cookies etc.).

2) What about a "response"? Is the response itself the parsed file.php file that gets sent back to the client machine?

Sort of. An HTTP response consists of a header text that describes: the response status (success or errors such as file not found, internal server error etc.), some content metadata (content type, encoding...) and the content.
The content could be an HTML document. It could also be a CSS or Javascript file, a PNG image or whatever other files the web server serves. The meta-data in the header describes the content in a way that the browser (or any client) can figure out how to handle it.

3) What if the file.php file contains a reference to a script.js file and a style.css file? At which stage do those files get served back to the client machine? Do they come in as separate headers or what?

Firstly, the process you just described would finish. Meaning, a request was sent and then a response was returned. Assuming the response is an HTML document, the browser parses the document and looks for external content: CSS stylesheets, Javascript files, image files, flash embeds and the like.
For each of these external files, the browser sends a new request using the exact same process. After getting a CSS file, for example, the browser knows to apply it to the document it just parsed.

4) Above in my answer, I'm not too sure if I was correct when I said "...because it has a .php extension, it first uses the PHP engine to parse any PHP code inside the file." Is that really the reason why the server parses the code inside the file, or does the server scan ALL kinds of files by default to check for any PHP code they might contain?

Well, it depends on the server configuration, but most of the times, yes;
The server is usually configured to handle all .php files the same, meaning pass them to the PHP parser and wait for its response.

By the way, this differs for different server-side software technologies. While this is the way PHP works, other technologies (e.g. Ruby on Rails, some .NET languages) are handled in a different way.

Great question, and good for you for showing interest!
For additional information, I suggest you check out HTTP on Wikipedia.

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Perfect "response" (pun intended). Thank you very much for your detailed and straight-forward explanation.
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A HTTP request looks like GET /index.html HTTP/1.1. It is sent as plain text to the web server.

A simplified HTTP response (with most header stuff removed) might look like this:

HTTP/1.1 200 OK Content-Length: 20 Content-Type: text/html; charset=UTF-8 <html>Hello</html>

If the page contains images or stylesheets or other external files, the web browser sends new requests for them, one request per file. The web server returns them in pretty much the same way it returned the HTML. When the browser has requested and received all the files it wants, the page is complete.

It is up to the web server to decide how it wants to process things like PHP. The browser doesn't need to know what goes on behind the curtains. From its perspective, it simply asks for content and (hopefully) receives it.

A simple web server might be configured to do exactly as you said. If it receives a request for a file ending in ".php", it would first run it through the PHP interpreter. But this is totally up to the web server owner to decide.

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I think this is what you are looking for: What really happens when you navigate to a URL

To summarize:
1. You enter a URL into the browser: facebook.com
2. The browser looks up the IP address for the domain name
3. The browser sends a HTTP request to the web server
4. The facebook server responds with a permanent redirect
5. The browser follows the redirect
6. The server ‘handles’ the request
7. The server sends back a HTML response
8. The browser begins rendering the HTML
9. The browser sends requests for objects embedded in HTML
10. The browser sends further asynchronous (AJAX) requests

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Thank you, will definitely check it out!

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