PHP Warning: Cannot modify header information - headers already sent by

PHP Warning: Cannot modify header information - headers already sent by - Solution

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Release： 2023-08-19 16:10:02

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PHP Warning: Cannot modify header information - headers already sent by - 解决方案

PHP Warning: Cannot modify header information - headers already sent by - Solution

In the process of using PHP to develop websites, sometimes we will encounter the following warning message: Cannot modify header information - headers already sent by (Cannot modify header information - header information has been sent by).

This warning message usually appears when we try to use the header() function to set HTTP header information in a web page. The root cause of this problem is that the content output to the browser already exists before the header() function is called. In other words, the header() function must be called before any output.

So, when we encounter this problem, how should we solve it? Here are a few common solutions.

Solution 1: Check whitespace characters and BOM

Whitespace characters and BOM (Byte Order Mark) are one of the common causes of "headers already sent" errors. Whitespace characters refer to characters such as spaces, newlines, or Tab keys at the beginning or end of the PHP file. The BOM is a special byte order used in UTF-8 encoding to mark the beginning of the file. The existence of these characters will be considered as output content, causing the header() function to be unable to be called normally.

To solve this problem, we can open the relevant PHP file and check whether there are blank characters or BOM at the starting and ending positions. If these characters are found, we should delete or replace them.

Solution 2: Avoid sending header before output

Since the header() function must be called before any output, when writing code, we should pay special attention to avoid calling header() function before sending any output. This includes the following situations:

Using output functions such as echo and print in PHP files
Including other files in PHP files that contain output statements
Functions or classes are defined in PHP files, and these functions or classes contain output statements

In order to avoid this situation, we can place the call to the header() function in The very beginning of the PHP file, ensuring no output occurs. In addition, we can also move the output statement after the header() function call to avoid interfering with the header() function call.

The following is a sample code that shows how to use the header() function correctly:

<?php
ob_start();
// 开启输出缓冲区

header('Content-Type: text/html; charset=utf-8');
// 设置HTTP头信息

// 其他代码...
?>

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>示例页面</title>
</head>
<body>
    <!-- 页面内容... -->
</body>
</html>

<?php
ob_end_flush();
// 输出缓冲区内容，并关闭输出缓冲区
?>

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By using the ob_start() and ob_end_flush() functions, we can buffer all output content, It will not be output until the header() function call is completed.

Summary:

When developing a PHP website, encountering the "Cannot modify header information - headers already sent by" warning message is a common problem. To solve this problem, we can do this by checking for the presence of whitespace characters and BOM, and avoiding calling the header() function before outputting. Hope these solutions help you!

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