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phpflushoutput-buffering

Is output buffering not working?

(I set output_buffering and implicit_flush to Off.)

I know PHP has its own output buffering. (Not output buffering that starts with ob_start())

<?php
echo 'hello';
header('HTTP/1.1 200 OK');
echo 'hello';
?>

So I expected that the above source code would work fine because of the output buffering unless I call flsuh().

However, a warning occurs. (Warning: Cannot modify header information - headers already sent by)

I do not know why. I want you to let me know if I have misunderstood.

Solution

  • You cannot output anything before setting headers.

    Functions that send or modify HTTP headers must be invoked before any output is made, regardless of php.ini settings.

    This is why sometimes the unintentional space before or after <?php, ?> will cause this error to be thrown.

    In any request, headers are sent first and then output. You are effectively outputting, then sending/modifying headers, and then outputting again.

    At the first invocation of any output, PHP will flush header buffers meaning they are already set, so trying to change them after is impossible; and that is why the error occurs.