Cannot have any output before sending header message
Any function that modifies or sends HTTP headers must be invoked before any form of output
Some functions modify HTTP headers:
Header/header_remove
session_start/session_regenerate_id
Setcookie/setrawcookie
And the main output may be as follows:
Implicit output
A space before or after the <?php or?>
BOM Header for UTF-8 file
Error messages that occurred before
Dominant output
Print, Echo, and other similar output functions
Raw HTML embedded between <?php
Why the Cannot modify header Information–headers already sent by error appears
To understand why the header message must be earlier than the output, let's look at the typical HTTP response. PHP primarily generates HTML content, but it also transmits a set of http/cgi headers to webserver:
http/1.1 OK
powered-by:php/5.3.7
Vary:donttrackmehere
content-type:text/html; Charset=utf-8
<body>
and <a href= "/" > </a>
The output is always a trailing header message. PHP must first send a header message to the Web server, and only one can be passed. You can no longer add any content to the header message after two lines are wrapped.
When PHP receives the first output (example: Print,echo,...), it refreshes and sends all header information for the setting. You can then output as you like, but you cannot modify the HTTP header information at this time.
How to find output older than a header message
The error message already contains all the information related to locating errors:
Warning:cannot Modify header Information-headers already sent by (output started at/www/usr2345/htdocs/auth.php:52) in /www/usr2345/htdocs/index.php on line 100
You can see that the header is invoked on the 100 lines of the file.
"Output started at" contains more important information: The 52 lines directly named in auth.php have produced output. That's what we need to find.
General situation:
Print,echo
The original HTML statement
<! DOCTYPE html>
<?php
The header message cannot be modified or sent.
Space before <?php
<?php
Here is a space
UTF-8 BOM
This is more difficult to detect but more prevalent. Be sure to save the file with your editor in "UTF-8 (no BOM)" mode.
The error message is shaped like "Unknown on line 0"
Generally, such an error indicates that the output is from the PHP extension or the php.ini set wrong message
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