Title: PHP Warning: Cannot modify header information Last modified: January 2, 2020 --- # PHP Warning: Cannot modify header information * Resolved [krazykatz911](https://wordpress.org/support/users/krazykatz911/) * (@krazykatz911) * [6 years, 5 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/) * I was getting this repeated PHP error on all my sites and very high memory usage. Once I deactivated this plug in, the errors stopped and memory good. * Here is the error code: * `PHP Warning: Cannot modify header information - headers already sent by (output started at /home/site/public_html/wp-cron.php:28) in /home/site/public_html/wp- includes/functions.php on line 6029` * Need a fix…. Viewing 12 replies - 1 through 12 (of 12 total) * Plugin Contributor [mbrsolution](https://wordpress.org/support/users/mbrsolution/) * (@mbrsolution) * [6 years, 5 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/#post-12287873) * Hi, * > I was getting this repeated PHP error on all my sites and very high memory > usage. Once I deactivated this plug in, the errors stopped and memory good. * Are you using the same theme and plugins in all your sites? * Thread Starter [krazykatz911](https://wordpress.org/support/users/krazykatz911/) * (@krazykatz911) * [6 years, 5 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/#post-12287984) * I have 10 sites, different themes. They were all experiencing this until I deactivated this plug in. - This reply was modified 6 years, 5 months ago by [krazykatz911](https://wordpress.org/support/users/krazykatz911/). * Plugin Contributor [mbrsolution](https://wordpress.org/support/users/mbrsolution/) * (@mbrsolution) * [6 years, 5 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/#post-12288002) * Do all the sites run in the same server, or different servers? If it is the same server, what are your server specs? * If they run in different servers, can you carry out the following troubleshooting steps for one of the sites. * Disable all Firewall rules in the plugin, then carry out a test. * Let me know what happens. * Thank you - This reply was modified 6 years, 5 months ago by [mbrsolution](https://wordpress.org/support/users/mbrsolution/). * Thread Starter [krazykatz911](https://wordpress.org/support/users/krazykatz911/) * (@krazykatz911) * [6 years, 5 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/#post-12288015) * Its all on the same server. 8 cores, 6gig memory. * Plugin Contributor [mbrsolution](https://wordpress.org/support/users/mbrsolution/) * (@mbrsolution) * [6 years, 5 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/#post-12288038) * Go to **Tools -> Site Health -> Status** in the admin panel of one of the sites. Let me know what it says? * Thank you * Thread Starter [krazykatz911](https://wordpress.org/support/users/krazykatz911/) * (@krazykatz911) * [6 years, 5 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/#post-12288050) * 17 items with no issues detected. All ok * Plugin Contributor [mbrsolution](https://wordpress.org/support/users/mbrsolution/) * (@mbrsolution) * [6 years, 5 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/#post-12288087) * Okay, lets try something different. * Do the following in one of the sites. Deactivate all of your other plugin except ours. Then carry out a test? If this does not help, test one of WordPress default themes like Twenty Nineteen. Let me know what happens. * Also do you have the latest WordPress version running? Are all of your plugins, theme up to date? * Thank you - This reply was modified 6 years, 5 months ago by [mbrsolution](https://wordpress.org/support/users/mbrsolution/). * Thread Starter [krazykatz911](https://wordpress.org/support/users/krazykatz911/) * (@krazykatz911) * [6 years, 5 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/#post-12290440) * I disabled all plugins on my sites, with just All in one running and error was happening. I just removed it from all my sites. I really don’t have time to start making test sites, etc. * Plugin Contributor [mbrsolution](https://wordpress.org/support/users/mbrsolution/) * (@mbrsolution) * [6 years, 5 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/#post-12291286) * Hi, sorry I was not able to help you further. * I am marking this thread as resolved since you are no longer running our plugin in your sites. * Kind regards * Thread Starter [krazykatz911](https://wordpress.org/support/users/krazykatz911/) * (@krazykatz911) * [6 years, 5 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/#post-12294589) * Ok, I researched this more and more, and found that this setting: Enable iFrame Protection * When enabled causes this repeated error. With it disabled, errors stop. * Plugin Contributor [mbrsolution](https://wordpress.org/support/users/mbrsolution/) * (@mbrsolution) * [6 years, 5 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/#post-12294780) * Thank you for reporting back. * It is interesting to know that the following feature **Enable iFrame Protection** is causing your issue. I am wondering if a plugin or theme is using iFrame in your site? * However, at least you now know what feature doesn’t work in your site. * Well done. * [Georg](https://wordpress.org/support/users/georgwordpress/) * (@georgwordpress) * [6 years, 4 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/#post-12365336) * [@krazykatz911](https://wordpress.org/support/users/krazykatz911/) Thank You very much!!!! * The same error on my sites – after disabling iFrame Protection in All in One the errors disappear. Viewing 12 replies - 1 through 12 (of 12 total) The topic ‘PHP Warning: Cannot modify header information’ is closed to new replies. * ![](https://ps.w.org/all-in-one-wp-security-and-firewall/assets/icon-256x256. png?rev=2798307) * [All-In-One Security (AIOS) – Security and Firewall](https://wordpress.org/plugins/all-in-one-wp-security-and-firewall/) * [Frequently Asked Questions](https://wordpress.org/plugins/all-in-one-wp-security-and-firewall/#faq) * [Support Threads](https://wordpress.org/support/plugin/all-in-one-wp-security-and-firewall/) * [Active Topics](https://wordpress.org/support/plugin/all-in-one-wp-security-and-firewall/active/) * [Unresolved Topics](https://wordpress.org/support/plugin/all-in-one-wp-security-and-firewall/unresolved/) * [Reviews](https://wordpress.org/support/plugin/all-in-one-wp-security-and-firewall/reviews/) ## Tags * [header](https://wordpress.org/support/topic-tag/header/) * [Modify](https://wordpress.org/support/topic-tag/modify/) * [php](https://wordpress.org/support/topic-tag/php/) * 12 replies * 3 participants * Last reply from: [Georg](https://wordpress.org/support/users/georgwordpress/) * Last activity: [6 years, 4 months ago](https://wordpress.org/support/topic/php-warning-cannot-modify-header-information-12/#post-12365336) * Status: resolved