The error "Cannot modify header information - headers already sent" appears because there is already output of content before attempting to send HTTP header information in PHP. 1. Check whether there are spaces or content at the beginning of the PHP file to ensure that there are no characters before <?php ; 2. Make sure that echo, print or include output is not used before calling header(), and the ob_start() buffered output can be enabled; 3. Check whether the file is in UTF-8 without BOM format to avoid hidden characters triggering output; 4. Turn off the display of debug information and fix all warnings and notices to prevent accidental output.
When the error "Cannot modify header information - headers already sent" appears, it means that when you try to send HTTP header information in PHP (such as using header() function), the content has been output to the browser. At this time, it is too late to modify the header information.
Although this problem seems scary, it is actually quite common, and the reason is basically concentrated on "the output that shouldn't have occurred in advance."
1. Check if there are spaces or content in front of <?php
This is the most common pitfall: if there are blank lines, spaces or HTML content at the beginning of your PHP file, these will be treated as output content, causing the header to fail to send.
Error example:
<?php header('Location: /');
Note that there is a blank line in front of <?php in the above code, which will cause the header to report an error.
?Solution:
- Open the file with the problem and check whether there are any characters at the beginning.
- If it is a pure PHP file, you can omit the
?>end tag to avoid accidentally adding spaces afterwards.
2. The output buffer is not turned on, and the content is directly echo or print
If you use echo , print , include to load HTML content, and even spaces and HTML tags before calling header() , the output will be triggered, causing the header to fail.
Error example:
echo "Login successfully"; header('Location: dashboard.php');
This code will report an error because echo has sent the content.
?Solution:
- Check your script logic to make sure there is no output before calling
header(). - You can use
ob_start()to enable output buffering before the script starts, delaying the output content.
3. Problems with file encoding or BOM header
Sometimes you see that the beginning of the file is fine, but you still report an error, which may be a problem with the file saving format. The encoding of UTF-8 with BOM inserts a hidden byte order mark (BOM) at the beginning of the file, and the PHP parser does not automatically ignore it, which is equivalent to outputting the content in advance.
?Solution:
- Use an editor (such as VS Code, Notepad) to save the file as UTF-8 without BOM format.
- Check whether multiple PHP files are combined with the BOM when outputting.
4. Debug information or warnings are output in advance
Sometimes, PHP error messages, warning or notice can also trigger output, especially when display_errors is enabled on your server.
for example:
Warning: Undefined variable $user in login.php on line 10
This type of warning itself will cause output, and it will also cause an error once it appears before header() .
?Solution:
- The development environment can enable error display, but it needs to be turned off after going online:
ini_set('display_errors', 0);
- Fix all warnings and notices to avoid unexpected output.
Basically these common situations. When encountering the header already sent error, priority is given to checking the output source. Solving the problem from the source is more effective than turning around.
The above is the detailed content of PHP header already sent error. For more information, please follow other related articles on the PHP Chinese website!
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