Description:
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SAPIs that do not support HTTP header(s), give warnings that output has been already sent like

"Warning: Cannot modify header information - headers already sent by..."

even those APIs do *not* support headers.

There is no use to warn about something that can never happen; There is nothing to modify; There are no headers that already have been sent.

It might make more sense to warn that this function is useless w/o HTTP response headers being available.

Compare with headers_sent(), it does differ between SAPIs.

Would be nice to have this a bit more streamlined.

Additionally header() could return FALSE if it failed like setcookie().

Test script:
---------------
In CLI:

<?php
echo "output";
session_start();
setcookie("TestCookie", 'value');
header('X-TestHeader: value');
?>