How to Fix "Warning: Cannot Modify Header Information" in WordPress pluggable.php
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Publisher ID: ca-pub-XXXXXXXXXXXXXXXX (Configured in config.php)When WordPress reports "Warning: Cannot modify header information - headers already sent by ... wp-includes/pluggable.php", the issue is never inside pluggable.php itself! It is caused by early whitespace in custom theme code.
Read the error message carefully: find the second file path mentioned under "output started at (file:line)". That file (typically your child theme's functions.php or a custom plugin) contains an echo statement, blank space, or UTF-8 BOM before "<?php". Fix that file, never edit pluggable.php!
📋 Guide Table of Contents
1. Problem Diagnosis & Symptoms
pluggable.php contains core functions like wp_redirect() and wp_set_auth_cookie(). When called, it fails if output already started.
2. Primary Root Causes
- Cause: Trailing spaces after closing ?> tag in theme functions.php.
3. Step-by-Step Diagnostic Fixes
Remove the Closing ?> Tag in functions.php
Edit functions.php and delete the trailing ?> tag at the end of the file.
4. How to Verify the Resolution
- Refresh the page; confirm warnings disappear and redirects proceed.
5. Diagnostic Troubleshooting Matrix
| Symptom Observed | Root Cause | Recommended Action |
|---|---|---|
| pluggable.php warning on line 1296 | Whitespace before <?php in child theme functions.php | Remove any blank lines or spaces before the opening <?php tag |
6. Critical Mistakes to Avoid
- ❌ Never edit core files in wp-includes/.
7. Frequently Asked Questions
❓ Why are pluggable functions called pluggable?
They allow plugins to override core functions before WordPress loads default implementations.
Official Technical References
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