Warning: Cannot modify header information

Thanks for opening a support topic. I’d be happy to troubleshoot this with you, or check this with the team if necessary. In order for me to do so, please share the following:

1. Your Site Health information. You can use this form to share privately if preferred.??
2. Has this started to occur for you recently, with no such warnings previously?

One we have the above we can perform some checks. This may include determining whether this occurs with only Site Kit active via the Health & Troubleshooting plugin, for which I can provide the steps involved.

Note also that this is a PHP warning and it shouldn’t impact any Site Kit functionality. It’s also very possible that this warning isn’t specific to Site Kit, as there are WordPress core features that also rely on the WordPress REST API, with information from various sources passed on each page load.

Let me know if you have any questions with the above.

I sent the info via the private form. As for the warning, it happened from the first time I activated site kit.

Appreciate the additional insights @googleman23. While I can’t determine the exact cause of the warning, it may be triggered by multiple parties, including Site Kit, all making requests on your wp-admin URLs. The warning doesn’t specifically state Site Kit, but we can try the following which would help determine if there are multiple plugins that are a factor in this warning appearing:

1. Login to your site from a Chrome browser incognito window
2. Install and activate the Health Check & Troubleshooting plugin.
3. Navigate to “Plugins > Health Check & Troubleshooting > Troubleshoot”. (This will temporarily deactivate other plugins and switch to a default theme – for you only, as a logged in user).
4. Check your debug logs or your PHP error logs once more (or set errors to display via WP_DEBUG_DISPLAY; true). Look for any warnings that appear from the moment you activate troubleshooting mode.
5. From back in your Health Check plugin click on the “Available Plugins” tab at the top right.
6. Enable your plugins one by one while rechecking does the PHP warning appear.
7. If you can’t locate the existing Analytics snippet try activating your present theme from within troubleshooting mode before checking does the warning appear.
8. At whatever stage you notice the warning re-appears, you’ll know which plugin you last activated, which may be conflicting with Site Kit.

Let me know if you have any questions with the above.

With regards the PHP warning not appearing when you apply WP_DEBUG_DISPLAY; false, this is normal, but the warning would appear in your debug logos and you PHP error logs.

As we didn’t receive a response I’ll mark this as resolved. Feel free to open a new support topic if you continue to encounter issues, or reopen this topic and we’d be happy to assist.