Cannot modify header information

This error also presents as "Cannot modify header information" depending on PHPhường version.

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In short, it means that somewhere in the code, something was printed khổng lồ the browser before wpuonline.com had finished preparing the page. This is most often caused by custom or modified code contributed from sources outside wpuonline.com, so inspect your uniquely added code (including themes) first..

If you get a "Headers already sent" error, there are three likely causes.

If this error is not the first error message on the page, then it is most likely a "avalanbịt effect" of previous errors & you may ignore it. Instead, focus on fixing the errors before it. When you fix the first error message(s), the "Headers already sent" error(s) will most likely disappear.

Text editors sometimes insert a UTF-8 byte order mark at the top of a file. In this case, the error message will usually say that "output started" at line 1 of some tệp tin. To fix this, configure your text editor lớn save the file without a byte order mark.

However, if you get an error "Headers already sent" as the first error and it tells you the error is near the kết thúc of a tệp tin (kiểm tra which file "output started at" points to), that probably means that there are extra spaces or lines after a closing ?> php tag. In wpuonline.com coding standards, it is strongly recommended (for this very reason) that PHP files should not have any closing ?> tags . Delete them, & everything should work fine.

Extra whitespace being added probably is caused by a bad unpacking program & / or a non-compliant editor (Windows Notepad or Wordpad, Mac TextEdit) adding it.

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Problems with "headers already sent" can also be caused by having a blank line at the kết thúc of *.inh files. wpuonline.com or more likely PHPhường seem lớn have sầu problems with extra spaces here and there.

Check all *.inc files lớn make sure there are no closing php ?> tags in any of them. Closing php ?> tags are not needed in your *.inch files. Also, kiểm tra all *.php files lớn make sure there are no blank lines at the beginning or at the kết thúc of the file.

If the error message indicates that this is caused by a module, disable modules one by one lớn find out which one is causing the problem.

This can also be caused by UTF-8. If a trang web is coded in ASCII và php files are being saved as UTF-8, it can cause this message. If the trang web và DB are both UTF-8, it should be ok khổng lồ save php files as UTF-8.

Additionally, this error message is related khổng lồ the "output_buffering" variable in php.ini. If output_buffering is mix to lớn some cabịt, the VPS will sover headers with delay (or modify them shortly after they are sent), & this error will not be tripped. But, if output_buffering is mix to lớn 0 or not at all, then headers can be sent at only one moment và, if there"s bad code, it will trip this error message. To sum up, turning on the "output_buffering" variable in php.ini fixes this problem.

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Hacked

Don"t get paranoid, but if you see this on a site that was previously working well, this could be a symptom of non-wpuonline.com code injection. It"s not uncomtháng for hackers who have sầu compromised a hệ thống to lớn run scripts that automatically inject HTML code into any *.php files they can find. Often by inserting spam liên kết into lớn page footers etc. Occasionally, this will have the side effect of breaking the code execution, or producing this error. Though rare, this error appearing on a previously stable site has sometimes led khổng lồ a hack being discovered.To demo for this fully, the only real solution is khổng lồ compare your current live code with the official version. In practice, checking *timestamps* on the files on the VPS can provide clues also. Search elsewhere for instructions on recovering from (& preventing) such hacks.