Title: PHP Warning:  Cannot modify header information
Last modified: August 30, 2016

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# PHP Warning: Cannot modify header information

 *  [onerock](https://wordpress.org/support/users/onerock/)
 * (@onerock)
 * [10 years, 9 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-9/)
 * The following PHP errors are filling up my log.
 * `PHP Warning: Cannot modify header information - headers already sent by (output
   started at /var/www/vhosts/***.com/httpdocs/wp-includes/cron.php:263) in /var/
   www/vhosts/***.com/httpdocs/wp-includes/pluggable.php on line 974`
 * Since these are both WordPress core files, I am absolutely sure they are not 
   the root cause. The order of the error, suggests that it is related to the cron.
   php affecting the pluggable.php.
 * My question is what direction do I begin troubleshooting? The pluggable.php suggest
   it is a plugin, but the error order seems to contradict this.
 * Thanks

Viewing 4 replies - 1 through 4 (of 4 total)

 *  Moderator [t-p](https://wordpress.org/support/users/t-p/)
 * (@t-p)
 * [10 years, 9 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-9/#post-6483174)
 * – Try reviewing this codex to solve header already sent problem: [http://codex.wordpress.org/FAQ_Troubleshooting#How_do_I_solve_the_Headers_already_sent_warning_problem.3F](http://codex.wordpress.org/FAQ_Troubleshooting#How_do_I_solve_the_Headers_already_sent_warning_problem.3F)
 *  Thread Starter [onerock](https://wordpress.org/support/users/onerock/)
 * (@onerock)
 * [10 years, 9 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-9/#post-6483180)
 * Yes, thanks you for the information link.
 * I had already investigated this article and chedked the wp-config.php just to
   be sure. I found that this was not the issue.
 * Also as previously noted, the error specifically references cron.php and pluggable.
   php files. These files are core files and should not be modified. There is something
   else is causing the issue and I am not sure what to try first.
 * Thanks again.
 *  Moderator [t-p](https://wordpress.org/support/users/t-p/)
 * (@t-p)
 * [10 years, 9 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-9/#post-6483188)
 * > There is something else is causing the issue and I am not sure what to try 
   > first.
 * Have you tried:
    – Flushing any caching plugins you might be running, as well
   as server and/or browser caches. Also, any op cache or content network cache,
   as well as Cloudflare, clear those caches. – deactivating ALL (yes all) plugins
   temporarily to see if this resolves the problem (plugin functions can interfere).
   If this works, re-activate them individually (one-by-one) to find the problematic
   plugin(s). – If you can’t get into your admin dashboard, try [resetting the plugins folder](http://codex.wordpress.org/FAQ_Troubleshooting#How_to_deactivate_all_plugins_when_not_able_to_access_the_administrative_menus.3F)
   by [FTP](http://codex.wordpress.org/FTP_Clients). Sometimes, an apparently inactive
   plugin can still cause problems. Also remember to deactivate any plugins in the
   mu-plugins folder (if you have created such folder). The easiest way is to rename
   that folder to mu-plugins-old. – switching to the unedited default Twenty Fifteen
   theme for a moment using the WP dashboard to rule out any theme-specific issue(
   theme functions can interfere like plugins). If you don’t have access to your
   admin area, use [FTP ](http://codex.wordpress.org/FTP_Clients), or your web-host’s
   cPanel or whatever file management application your host provides. Navigate to/
   wp-content/themes/ and switch to the default theme by renaming your current theme’s
   folder by adding “-old” to the end of the folder name. Alternately, you can remove
   other themes except the default theme. That will force your site to use it. –
   If all the above steps do not resolve the issue, then try **MANUALLY** updating.
   Download a fresh copy of the WordPress .zip file to your computer, unzip it, 
   and use that to copy up all files and folders **EXCEPT the wp-config.php file
   and the /wp-content/ directory.** You may need to delete the old wp-admin and
   wp-includes folders and files on your server before uploading the new ones. Read
   the [Manual Update ](http://codex.wordpress.org/Updating_WordPress#Manual_Update)
   directions first.
 *  Thread Starter [onerock](https://wordpress.org/support/users/onerock/)
 * (@onerock)
 * [10 years, 9 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-9/#post-6483197)
 * Thank you for the troubleshooting steps. I will diligently try each one of these
   to see if the problem is resolved.

Viewing 4 replies - 1 through 4 (of 4 total)

The topic ‘PHP Warning: Cannot modify header information’ is closed to new replies.

## Tags

 * [PHP Warning](https://wordpress.org/support/topic-tag/php-warning/)

 * In: [Fixing WordPress](https://wordpress.org/support/forum/how-to-and-troubleshooting/)
 * 4 replies
 * 2 participants
 * Last reply from: [onerock](https://wordpress.org/support/users/onerock/)
 * Last activity: [10 years, 9 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-9/#post-6483197)
 * Status: not resolved

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