Tips to avoid outputting anything before using header()

M66 2025-05-28

In PHP programming, the header() function is used to send raw HTTP header information. Common application scenarios include redirecting users, setting cache control headers, or modifying content types. However, there is an important limitation to the use of the header() function - there cannot be any output before calling header() . Otherwise, PHP will not send header information correctly and may result in an error.

Why do you need to avoid outputting content?

Requirements for HTTP protocol
The HTTP protocol stipulates that header information must be sent before the response body. Therefore, when using the header() function, you must make sure that nothing has been output to the browser. Any output (including HTML, spaces, line breaks, or error messages, etc.) will cause PHP to send a part of the response body, thus preventing subsequent header information from being sent. In other words, header() must be called before any output.
"Cannot modify header information" error If anything is output before calling the header() function, PHP will throw an error: "Warning: Cannot modify header information - headers already sent by...". This means that PHP has started sending response content and the header information can no longer be modified or sent. This error usually disrupts the normal function of the website, especially in scenarios such as redirection and login verification.
Cache Control
When PHP sends header information, you can use header() to control cache, type, or redirection. If there is output, the HTTP header cannot be set as expected, which may result in undesired caches or page redirection fails to be properly performed.

How to avoid this problem?

Make sure there are no blank outputs Avoid any whitespace characters or line breaks at the beginning and end of a PHP file. Typically, this problem occurs at the beginning or end of a PHP file, especially before or after the < ? php tag. Make sure there are no spaces or line breaks at the beginning and end of the file.
For example, the following code creates problems:
```
 <?php
// mistake：There are blank characters in front
header("Location: http://m66.net");
exit;
?>
```
The correct way is:
```
 <?php
header("Location: http://m66.net");
exit;
?>
```
Use ob_start() and ob_end_flush()
PHP provides an Output Buffering mechanism that allows capturing all output content before sending an actual response. By using ob_start() , PHP starts buffering output until ob_end_flush() is called. This allows you to call the header() function before outputting anything.
Example:
```
 <?php
ob_start(); // Start output buffering
header("Location: http://m66.net"); // Now safe to use header
ob_end_flush(); // Clear the buffer and send the output
exit;
?>
```
Avoid outputting error messages During development, PHP usually displays a warning or error message if an error occurs. If error messages are output, they will affect the sending of the header. To avoid this, you can disable the error output by configuring the php.ini file or using the ini_set() function.
Configure php.ini :
```
 display_errors = Off
```
Use ini_set() in your code:
```
 <?php
ini_set('display_errors', 0); // 禁止mistake输出
header("Location: http://m66.net");
exit;
?>
```
Check third-party libraries or frameworks Some third-party libraries or frameworks may automatically output content or perform debugging during request processing. At this time, you can check the documentation or source code of these libraries to make sure that they do not output content before calling header() . Some frameworks allow you to configure log levels or output levels to avoid this problem.
Debug output During the development stage, if you need to debug, you can temporarily use ob_start() for output buffering, or comment out all outputs after debugging is completed to ensure that the header information is sent before output.

Summarize

When using PHP's header() function, be sure to make sure that nothing is output before it is called. Spaces, line breaks, HTML code, or error messages, etc. can cause PHP to fail to send header information correctly. By rationally using output buffers, avoiding output error information, and carefully scheduling the code structure, this problem can be effectively avoided, ensuring that the header() function works normally, thereby achieving correct redirection, cache control and other functions of the website.