The error "Cannot modify header information - headers already sent" appears because there is already output of content before trying to modify the header in PHP. 1. Check whether there are spaces or line breaks at the beginning of the PHP file, and make sure to start with <?php ; 2. Make sure that there is no output such as HTML or echo statements before the call of functions such as header(), session_start(); 3. Avoid unnecessary ?> tags and subsequent blanks at the end of the file; 4. Check whether the included files have introduced unexpected output, and you can use the ob_start() buffer to control the output.
When the error "Cannot modify header information - headers already sent" appears, it means that when you try to modify header information in PHP (such as using header() , setcookie() or session_start() ), the content has been output. The PHP header must be sent before any actual output, so once the output starts, it cannot be modified.
1. Check if there are spaces or output appears in front of <?php
This is one of the most common reasons. If your PHP file has blank lines, spaces or even newline characters at the beginning, it will be treated as output content.
// Error example: There is space or line break in front of it <?php session_start(); ?>
? The correct way to do this is to make sure your PHP file starts with <?php and does not have any characters in front of it, including blanks and comments.
- Use the code editor to view hidden characters (such as displaying whitespace characters)
- Delete all spaces and empty lines at the beginning of the file
- If you use the template structure, you should also pay attention to whether the main entry file starts cleanly
2. Check whether there is any content output before header() call
Sometimes it is not the beginning problem, but the content is output in advance somewhere in the middle. for example:
- Write HTML content directly
- Use
echo,print,var_dump - File containing a certain output content (such as debug statement not deleted)
<?php echo "Debug info"; // The content has been output here header('Location: /'); // So will this line report an error?>
? Solution:
- Put all
header()andsession_start()before the output logic - Avoid using debug output before these function calls
- You can use
ob_start()to start the buffer to delay output (suitable for complex projects)
3. Pay attention to the end of the file ?> Is it redundant
Many PHP files do not need to close the tag in the end ?> , especially pure PHP files. Because new lines or spaces after closing tags are also taken as output.
<?php // some code... ?>? ← This line break will be treated as output!
? Best Practices:
- Pure PHP file omitted
?>End tag - If you have to keep it, make sure there are no line breaks or spaces after it
4. Check whether the included files have introduced unexpected output
Sometimes the problem lies in your include or require file. for example:
- A configuration file accidentally hits an empty line
- There is a class file at the end of it
?>and it has a line break - There is debug output in third-party libraries
? Solution:
- Check the include files one by one
- Use
ob_get_contents()to see if there is any content in the buffer output in advance - Add
ob_start()at the top of the entry file to avoid such problems affecting the header
Basically, these common causes and solutions. Although this type of error seems simple, it may be a bit annoying to troubleshoot, especially when multiple files are nested in include in large projects. Keeping the code clean and familiar with the output process can reduce pitfalls.
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