Where the headers come from when just calling echo in PHP? - php

Question

I see come code from PHP manual
$fp = fsockopen("www.example.com", 80, $errno, $errstr, 30);
if (!$fp) {
echo "$errstr ($errno)<br />\n";
} else {
$out = "GET / HTTP/1.1\r\n";
$out .= "Host: www.example.com\r\n";
$out .= "Connection: Close\r\n\r\n";
fwrite($fp, $out);
while (!feof($fp)) {
echo fgets($fp, 128);
}
fclose($fp);
}
writing header by echo, is this going to work?
when just calling simple code like this:
echo 'hello';
where the http headers from?

Answer 1

What you're doing in this code is:
Open a socket connection to a remote HTTP server (www.example.com on port 80). This establishes a TCP connection to that port.
You then send (via fwrite) an HTTP request over this connection. HTTP is a protocol on top of TCP, and you're manually formulating your HTTP protocol headers here.
You're then reading (via fgets) the (HTTP) response of the remote server.
I'm guessing that you want to know why you see HTTP headers in this remote response, even though you're only doing echo 'hello'; on the remote server. The answer is because the web server running on that server is handling the HTTP transaction. You're not handling any details of the incoming HTTP request in PHP, and you're also not handling any details of the outgoing response. The web server on which PHP is running (likely Apache) is doing that.
The whole stack includes a TCP connection, which carries an HTTP request, which consists of HTTP headers and an HTTP body. On the server, the TCP connection is typically handled by the underlying operating system, which makes the connection available as a socket to the web server, where the web server "unwraps" the HTTP request to handle it and invoke PHP if necessary, and then the whole chain goes backwards for the response.