Notice being printed out, that also counts as premature output. If another PHP statement or expression causes a warning message or Generating an implicit PHP startup/warning message.
But it could also be any doubly loaded extension= module. 
It's occasionally the gzip stream encoding setting. It's typically a PHP extension or php.ini setting if no error source Error source mentioned as "Unknown on line 0". (Quite commonly include()d scripts are the culprit.) This eschews a small portion of these cases. It's commonly advised, in particular to newcomers, that trailing ?> PHPĬlose tags should be omitted. Any text/space characters after it will be written out as page content The PHP end marker does not terminate script execution at this point. Then this is where some whitespace or the raw text got written out. If the error source is mentioned as behind the Script conditions that will trigger a header() call must be noted 
readfile, passthru, flush, imagepng, imagejpeg. trigger_error, ob_flush, ob_end_flush, var_dump, print_r. Ensure header() calls occur before messages The application flow must be restructured to avoid that. Intentional output from print and echo statements will terminate the opportunity to send HTTP headers. That's where you had to look for premature output. In this example, it's auth.phpĪnd line 52. It denominates the source of previous output. The " output started at" note within the parenthesis is more significant. Here "line 100" refers to the script where the header() invocation failed. www/usr2345/htdocs/index.php on line 100 (output started at /www/usr2345/htdocs/ auth.php:52) in Warning: Cannot modify header information - headers already sent by The header() warning contains all relevant information to How can you find out where the premature output occurred? But sending further HTTP headers is impossible then. When PHP receives the first output ( print, echo, ) it willįlush all collected headers. It can only do that once.Īfter the double linebreak it can nevermore amend them. The page/output always follows the headers. Raw sections prior PHP page output page. print, echo and other functions producing output. Some functions modifying the HTTP header are: Warning: Cannot modify header information - headers already sent (output started at script:line) Functions that send/modify HTTP headers must be invoked before any output is made.
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