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The "Cannot modify header information - headers already sent by" error is among the most common issues encountered by PHP developers. This problem arises when a script attempts to send an HTTP header after output has already been sent to the browser. In PHP, headers must be sent before any output. The error message typically includes the file and line number where the output was initiated. This article provides a comprehensive overview of the potential causes and solutions for this issue.

Causes of the Problem

The "Cannot modify header information" issue can stem from several causes:

• Output before sending headers: The most common cause is output (echo, print, HTML code outside PHP tags, etc.) occurring before calling functions that manipulate headers (e.g., header(), session_start(), setcookie()).

• BOM (Byte Order Mark): Files saved with a BOM (especially in UTF-8 format) can cause invisible output to be sent before the start of the PHP code.

• Whitespace: Unintended spaces or empty lines before <?php or after ?> can lead to output being sent before the headers.

Solutions to the Problem

To fix the "Cannot modify header information" error, several approaches can be utilized:

1. Remove output before headers: Ensure no output occurs before calling functions that send headers. This includes checking for whitespace (spaces and blank lines) before <?php and after ?>, removing BOM, and ensuring no HTML or echo statements precede header manipulations.

2. Omit closing PHP tags: In files that contain only PHP code, it is recommended to not end the file with a closing ?>. This helps prevent unintended output caused by whitespace after the PHP tag.

3. Use output buffering: PHP offers output buffering, which allows you to start sending output to a buffer instead of directly to the browser. This enables you to manipulate headers even after some output has already been generated. Output buffering can be enabled by calling ob_start() at the beginning of the script.

The "Cannot modify header information - headers already sent by" error can be frustrating, but it's usually solvable with a bit of attention to detail in your code. Understanding how PHP processes output and headers, and adhering to best practices for their proper manipulation, is key to resolving this issue. By fixing this problem, not only will you improve the functionality of your application, but you will also avoid potential security risks and enhance the overall quality of your code.