According to the PHP documentation:
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.
but when I tried the example that the documentation reports (with a little change):
<html>
<?php
/* This will give an error. Note the output
* above, which is before the header() call */
header('X-Header: http://www.example.com/');
exit;
?>
all worked just fine, no error poped up and I smoothly got my <html> tag in the output and my X-Header in the headers.
I'm using PHP 7.1.9, so is still correct what the documentation says?
The documentation is still correct.
For performance purposes, the interpreter puts the output in a buffer. When the buffer is filled for the first time, it sends the headers before sending the content of the buffer.
This lets the script produce a small amount of output before sending the headers.
Read more about output buffering configuration settings.
The option output_buffering allows turning the feature off or on and even setting the size of the buffer.
The option implicit_flush tells the interpreter to flush the buffer after every output block. This forces your script to send the headers correctly, before any output.
edit your php.ini and enable Output Buffering ..