Fix WordPress Warning: “Cannot modify header information – headers already sent…”
The “Cannot modify header information – headers already sent” warning is a common issue in PHP and can occur in WordPress. This warning typically arises when your PHP code attempts to send a HTTP header to the client but there’s already output being sent to the client.
Usually, this problem is caused by:
1. PHP closing tag (`?>`) at the end of your files
2. Whitespace or blank lines before opening `<?php` or after closing `?>` PHP tags
3. BOM (Byte Order Mark) issue with UTF-8 encoded file
4. Direct echo/print output before header() or setcookie() function
To fix this error, you need to find which file is causing it. The warning message will tell you the specific file and line number.
Follow these steps:
1. **Open the indicated file:** The file will usually be in your WordPress directory, in either the wp-content/themes or wp-content/plugins directory.
2. **Find the indicated line number:** Scroll down to the line number that was specified in the error message. The issue is often near the beginning of the file, before a header() or setcookie() call.
3. **Check for output before header() or setcookie():** Ensure that no echo, print, or other functions that send output to the client are called before header() or setcookie(). This also includes HTML, like `<!DOCTYPE html>` or `<html>`, and whitespace or blank lines.
4. **Remove closing PHP tag:** If the file ends with `?>`, it’s usually safe to remove. PHP doesn’t require closing tags and removing it prevents accidental output caused by trailing whitespace.
5. **Check for BOM:** If you’re using a text editor like Notepad++, you can check if a file includes a Byte Order Mark (BOM). To check, open the file in Notepad++, then click on the “Encoding” menu and see if “Encode in UTF-8-BOM” is ticked. If it is, click on “Convert to UTF-8 without BOM”, and then save the file.
6. **Save your changes and re-upload the file:** If you’re editing the file via FTP, make sure to re-upload the file after making your changes. Then, refresh your WordPress site to see if the error has been resolved.
If you’re uncomfortable making these changes or you can’t find the problematic file, you may want to consider reaching out to a professional developer or the support team for your web host. Always remember to backup your site before making any changes.