Why does header() report an error "Headers already sent"? Causes and solutions

M66 2025-06-05

In PHP, the header() function is used to send raw HTTP header information. It is usually used for redirection, setting page content type, cache control and other operations. However, when using the header() function, you often encounter an error message: "Headers already sent". This article will explain in detail the causes and solutions to this problem.

What is the "Headers already sent" error?

When you use the header() function in a PHP script, it needs to send header information before any output (such as HTML content, spaces, line breaks, etc.). This error will result if the PHP script has sent the content (i.e., any data is output before calling the header() function). An error message usually prompts "Headers already sent", which means that the header message has been sent to the browser and cannot be sent again.

Why does this error occur?

Common causes of "Headers already sent" errors include:

HTML or spaces are output in advance :
This error will be triggered if any HTML content, spaces, or newlines have been output in the PHP script before calling the header() function. Especially spaces or newlines outside PHP tags ( <?php ?> ) are easily overlooked.
File encoding issues :
If the encoding format of the PHP file is UTF-8 and the file contains a BOM (byte order mark), then the BOM will be considered as output even if the output content is not displayed at the top of the file, resulting in the header information being sent in advance.
Output buffer problem :
In some cases, PHP's output buffer settings may be inappropriate, resulting in content output in advance. For example, if you do not start the output buffer with ob_start() , PHP may send part of the output before calling header() .

How to resolve the "Headers already sent" error?

Make sure there is no output before the header() call :
You need to check the code before the header() call to make sure there is no HTML output or whitespace characters. Even a space or a newline throws this error. The best thing to do is to place the header() function at the top of the PHP script to make sure there is no output.
Error example :
```
 <html>
<body>
<?php
header("Location: https://m66.net/somepage.php");  // This ban will report an error
?>
</body>
</html>
```
Correct example :
```
 <?php
header("Location: https://m66.net/somepage.php");  // Use correctly
exit();
?>
<html>
<body>
</body>
</html>
```
Check the file encoding and remove the BOM :
If your file is UTF-8 encoded and contains BOM, you can resave the file through a text editor (such as Sublime Text, Notepad++) and select the BOM format without it. In addition, when reading files using file_get_contents() or fopen() , you should also make sure that there is no BOM included.
Enable output buffer :
You can enable the output buffer at the beginning of the PHP script and start the buffer through ob_start() so that even if there is output, it can be temporarily stored in the buffer until you manually call ob_end_flush() or the script ends.
Example :
```
 <?php
ob_start();  // Start output buffering
header("Location: https://m66.net/somepage.php");  // This line will not report any errors
ob_end_flush();  // Output buffer content
exit();
?>
```
Check the imported files :
If you use include or require to introduce other PHP files, you need to make sure that these files do not call the header() function before outputting anything. You can place the header() call before all other PHP code to make sure it is not interfered with by the output content.
Debugging tools :
Using the headers_sent() function can help you diagnose problems. This function will tell you where and when the header message has been sent. You can use it to find out where the error is.
Example :

header