Description:
------------
With fastcgi.error_header set to some value (e.g. "HTTP/1.1 500 Internal Server Error"), PHP-FPM will attempt to set the header value when the PHP exit status is determined to be 255 - https://github.com/php/php-src/blob/f89ee8040a8b8d93981c9df85a79592ceac02f1f/sapi/fpm/fpm/fpm_main.c#L1950-L1958

If headers have already been sent, then an E_WARNING is triggered by sapi_header_op - https://github.com/php/php-src/blob/f89ee8040a8b8d93981c9df85a79592ceac02f1f/main/SAPI.c#L675-L686

As such, if headers have been sent (e.g. due to output), then at some point PHP sets the exit_status value to 255 (e.g. due to an error), and fastcgi.error_header is set to some value, then an E_WARNING will be triggered.

This seems undesirable - perhaps it should simply not attempt to call sapi_header_op if headers have already been sent?

Test script:
---------------
<?php

// With fastcgi.error_header set to "HTTP/1.1 500 Internal Server Error"

echo 'x';

register_shutdown_function(function() {
    file_put_contents('/tmp/error', print_r(error_get_last(), true));
});

z(); // trigger a fatal error

Expected result:
----------------
A fatal error will be triggered, and the last error visible when shutting down will be the fatal.

e.g.

$ cat /tmp/error
Array
(
    [type] => 1
    [message] => Call to undefined function z()
    [file] => .../test.php
    [line] => 7
)

Actual result:
--------------
PHP-FPM attempts to replace the header, but headers have already been sent (due to output):

$ cat /tmp/error
Array
( 
    [type] => 2
    [message] => Cannot modify header information - headers already sent by (output started at test.php:3)
    [file] => Unknown
    [line] => 0
)