Why must the XHR status be set before echoes in PHP?

Question

Perhaps this question belongs on Stack Overflow or another site instead, in which case please tell me and don't just downvote; I will gladly move it.

Anyway, I'm just learning the very basics of server management and I was playing around with my own small, private MySQL server on my laptop when I happened upon something I hadn't expected. When I sent an AJAX request to a PHP page using the jQuery $.ajax() method, I was continuing to get the response code of "200" even though I called the function http_response_code(500); in my PHP code. I couldn't figure out why this was happening until I realized that I was using echo statements to return data to the Javascript page, and by moving the http_response_code(500); function to above all of the echo statements, it would set the HTTP response code correctly.

I understand this is correct functionality, I am just wondering why this is the case? Why must the HTTP response code be set before using echo functions? What's happening in the underlying protocols? My guess is that it has something to do with echo forcing the script to return data when it is called, but then why do scripts like the following:

echo "thing one";
echo "thing two";
http_response_code(500);
echo "thing three";

...still echo "thing three" just fine but don't change the response code?

I'm not having a "problem" per se, I'm just here to learn! Thanks!

Answer 1

When you print anything on the body, headers must be already sent. That's if you e.g. echo() something or even if there's a space before the <?php. A web server simply can't output anything before it has sent the headers, as that's the form of a HTTP response, causing any output to send the headers, too.

If you try to use e.g. header() that sends a header, you could get a Warning: Cannot modify header information - headers already sent by.

Remember that header() must be called before any actual output is sent, either by normal HTML tags, blank lines in a file, or from PHP. It is a very common error to read code with include, or require, functions, or another file access function, and have spaces or empty lines that are output before header() is called. The same problem exists when using a single PHP/HTML file.

Because http_response_code() does not send HTTP headers, but only prepares the header list to be sent later on, it won't trigger this error. While using headers_sent() in an if condition can prevent these errors with header(), it could also be useful for debugging issues when using http_response_code().