When encountering the "Cannot modify header information – headers already sent" error, you should first find the location where the output starts, and then check and clear the redundant output source. The specific steps are as follows: 1. Position the output starting file and line number according to the error message; 2. Check whether there are echo, print and other output statements or HTML content before and after this location; 3. Check whether there are whitespace characters or closed tags at the beginning and end of the PHP file?>; 4. Use ob_start() to temporarily buffer the output to debug the code; 5. Troubleshoot hidden output sources such as imported files, UTF-8 BOM header, and php.ini configuration.

In PHP development, if you encounter an error like "Cannot modify header information - headers already sent", it usually means that before you call header() , setcookie() and other functions, the content has been output to the browser. To solve this problem, you must first find the "where the output starts", that is, where the output is generated before calling header() .

Check the approximate location of the error message

PHP error message usually tells you "headers already sent in /path/to/file.php on line X". This prompt is very important, it indicates which line of the file has started the output.

for example:

 Warning: Cannot modify header information - headers already sent by (output started at /var/www/index.php:10) in /var/www/redirect.php on line 20

This means that header() is called on line 20 of redirect.php , but the output has already started as early as line 10 index.php .

• Check whether there are echo , print , var_dump and other output statements before and after the line
• Note whether you accidentally wrote spaces or HTML outside the PHP tag

Check for whitespace characters and closed tags ?>

A common but easily overlooked problem is that there are extra whitespace characters (including newlines) at the end or multiple places of the PHP file, or do you use a closed tag ?> and add spaces after it.

For example:

 <?php
// some code here

?>
 (There is an invisible space or line break here)

In this case, PHP will treat the blanks after the closed tag as output content, thereby sending HTTP header information in advance.

suggestion:

• If the file contains only PHP code, don't write closed tags ?>
• Use the editor to display invisible characters, check whether there are spaces or line breaks at the beginning and end
• Use ob_start() to buffer the output as a temporary solution (but it does not cure the symptoms)

Control ob_start() using output buffering

If you cannot find the output starting point immediately, you can use the output buffer to delay the output, so that even if there is advanced content, it will not be sent to the browser immediately.

 <?php
ob_start(); // Turn on output buffering// Your code logic, including header()

ob_end_flush(); // Finally refresh the buffer output

Although this is not the fundamental solution, it can help you bypass the error during the debugging phase and allow you to continue running the code.

Notice:

• ob_start() must be called before any output
• It is not recommended to rely on this method for a long time, but the source problem should be fixed.

Find hidden output sources

Sometimes the output does not come directly from the code you wrote, but in the following situations:

• There is unexpected output in the imported PHP file (such as echo for debugging)
• UTF-8 BOM header causes output (automatically added when saved by some editors)
• php.ini configuration enabled incorrectly (such as output_buffering=On or expose_php=On )

Troubleshooting suggestions:

• Check all include/require files, especially configuration files or public function libraries
• Open the file with a hexadecimal viewer and confirm that there are no hidden BOM characters (EF BB BF)
• Turn off unnecessary output options on the server

Basically that's it. This kind of problem looks scary, but in fact, you can find the reason by checking the error position, whitespace characters, introducing files and BOM in order. The key is to develop good coding habits, such as not adding closed tags at the end of PHP files, avoiding outputting content before header, etc.

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