How to solve PHP Warning: Cannot modify header information - headers already sent by output started at

When developing PHP applications, you often encounter a warning message "Cannot modify header information - headers already sent by output started at". This warning message usually causes the application to break, affecting the user experience. This article will explain why this warning occurs and provide some solutions.

First, let’s understand what this warning message means. "Cannot modify header information" means that some output content has been sent to the browser before sending the HTTP header information to the browser. Normally, HTTP header information is set using the header() function at the front of the PHP script, such as setting the response content type, redirection, etc. However, any preceding output (including spaces, newlines, error messages, etc.) will result in this warning message.

The specific content of the warning message will generally include "output started at", which indicates which line of which file started outputting the content. This tip can help us locate the problem.

So, how to solve this problem? Here are a few common solutions.

1. Check the file encoding and file format: Make sure the encoding format of the PHP file is UTF-8 and there is no BOM (Byte Order Mark) mark. Sometimes, some editors automatically add a BOM tag at the beginning of the file, which causes the output content to be sent to the browser, triggering a warning. You can use a specialized text editor, such as Notepad, to check and modify the file encoding.
2. Check for spaces and line breaks: Make sure there are no spaces or line breaks before the <?php tag. Any spaces or newlines before the header() function will be treated as output content and trigger a warning.

The following is a sample code that demonstrates the common causes and solutions to this problem:

<?php
// 錯誤示例 - 會產生警告
echo "Hello World!";
header("Location: http://example.com");
exit;

// 解決方案 - 移除輸出內容前的空格和換行
<?php
header("Location: http://example.com");
exit;

// 解決方案 - 使用輸出緩沖區(qū)
<?php
ob_start(); // 啟動輸出緩沖區(qū)
echo "Hello World!";
header("Location: http://example.com");
exit;
ob_end_flush(); // 刷新緩沖區(qū)并發(fā)送內容給瀏覽器

// 解決方案 - 修改文件編碼和格式
<?php
ob_start(); // 啟動輸出緩沖區(qū)
echo "Hello World!";
header("Location: http://example.com");
exit;
ob_end_flush(); // 刷新緩沖區(qū)并發(fā)送內容給瀏覽器

// 解決方案 - 使用die()或exit()函數代替header()函數
<?php
echo "Hello World!";
die("Location: http://example.com");
?>

In summary, when "Cannot modify header information - headers already sent by" appears output started at" warning message, we can check the file encoding and file format, remove spaces and newlines before the output content, use the output buffer, or use die() or exit () function to solve the problem. It is important to note that there is no output before the header() function to avoid this warning. In this way, we are able to provide a better user experience and ensure the normal operation of the application.

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