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The error "Warning: session_start(): Cannot send session cache limiter - headers already sent" is a common issue encountered by developers working with PHP. This warning typically occurs when a script attempts to start a session after output has been sent to the browser, such as HTML, whitespace, or an echo statement. In PHP, it's necessary to start sessions before sending any output because session data is part of the HTTP headers. In this article, we'll explore how to identify and fix this issue.

Causes of the Error

The error can be caused by several factors:

• Whitespace before the PHP opening tag (<?php): Unintended spaces, new lines, or other types of whitespace before the PHP tag can cause output to be sent before session_start().

• Echo or print statements before session_start(): Any output generated by echo or print statements before calling session_start() will cause this warning.

• BOM (Byte Order Mark) in UTF-8 files: Some text editors add an invisible BOM character at the start of files, which can lead to the error mentioned above.

Solving the Error

1. Verify output before the PHP tag: Check if there are any white spaces before the opening PHP tag. These spaces can also be at the end of files if your script is divided into multiple files.

2. Omit the closing PHP tag in purely PHP files: It's recommended to omit the closing PHP tag (?>) at the end of files containing only PHP code to prevent unintended output.

3. Check echo and print statements: Ensure that echo, print, or any other output-generating functions are not used before calling session_start().

4. Fixing the BOM issue: If the problem is caused by BOM, use a text editor capable of displaying and removing BOM.

5. Output buffering: As a last resort, you can use output buffering by calling ob_start() at the beginning of your script. This function captures all output until you explicitly call ob_end_flush() or ob_end_clean(), allowing you to start a session even after some output has been sent.

Summary

The "Warning: session_start(): Cannot send session cache limiter - headers already sent" error is often caused by unintended output before starting a session. By verifying and removing whitespace, unnecessary output before session_start(), and any potential BOM, you can effectively resolve this problem. If none of the previous solutions work, wrapping output using output buffering typically eliminates the issue.