PHP header redirect working in nginx even output sent

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http://serverfault.com – note: There is a similar question but that's about Apache. The following code <?php echo '.'; header('Location: http://stackoverflow.com'); is fully 'working' under nginx 0.7.67 where 'working' means the php redirect is working fine without errors in the error log or displaying the .. How can i force nginx or php-fpm to throw the PHP Warning: Cannot modify header information - headers already sent by warning? (HowTos)