Title: Error Warning: Cannot modify header information
Last modified: October 9, 2020

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# Error Warning: Cannot modify header information

 *  [periago89](https://wordpress.org/support/users/periago89/)
 * (@periago89)
 * [5 years, 8 months ago](https://wordpress.org/support/topic/error-warning-cannot-modify-header-information-8/)
 * Buenas tardes, estoy teniendo el siguiente problema.
 * En mi tienda online, he introducido la siguiente línea en el functions.php para
   eliminar el típico mensaje de “se ha añadido al carrito X producto”
 * La línea es la siguiente:
 * /* Quitar mensaje de “Producto X se ha añadido a tu carrito” */
 * add_filter( ‘wc_add_to_cart_message_html’, ‘__return_null()’ );
 * El problema es que al añadirlo,cuando añado un producto al carrito me saltan 
   dos mensajes de error en la parte superior de la pagina:
 * Warning: call_user_func_array() expects parameter 1 to be a valid callback, function‘
   __return_null()’ not found or invalid function name in /home/customer/www/perlighting.
   com/public_html/wp-includes/class-wp-hook.php on line 289
 * y
 * Warning: Cannot modify header information – headers already sent by (output started
   at /home/customer/www/perlighting.com/public_html/wp-includes/class-wp-hook.php:
   289) in /home/customer/www/perlighting.com/public_html/wp-content/themes/generatepress/
   header.php on line 81
 * La cuestión es que he quitado la línea del functions.php y el primer mensaje 
   ha desaparecido, pero ahora me sale otro mensaje distinto a los anteriores que
   no se como arreglar y es el siguiente:
 * Warning: Cannot modify header information – headers already sent by (output started
   at /home/customer/www/perlighting.com/public_html/wp-content/themes/generatepress/
   header.php:12) in /home/customer/www/perlighting.com/public_html/wp-content/themes/
   generatepress/header.php on line 81
 * Agradecería cualquier ayuda, ya que me estoy volviendo loco.
 * The page I need help with: _[[log in](https://login.wordpress.org/?redirect_to=https%3A%2F%2Fwordpress.org%2Fsupport%2Ftopic%2Ferror-warning-cannot-modify-header-information-8%2F%3Foutput_format%3Dmd&locale=en_US)
   to see the link]_

Viewing 3 replies - 1 through 3 (of 3 total)

 *  [Joy](https://wordpress.org/support/users/joyously/)
 * (@joyously)
 * [5 years, 8 months ago](https://wordpress.org/support/topic/error-warning-cannot-modify-header-information-8/#post-13514550)
 * > Good afternoon, I am having the following problem.
   >  In my online store, I have
   > introduced the following line in the functions.php to eliminate the typical
   > message of “X product has been added to cart” The line is the following:
   >     ```
   >     /* Remove message from "Product X has been added to your cart" */ 
   >     add_filter ('wc_add_to_cart_message_html', '__return_null ()');
   >     ```
   > 
   > The problem is that when I add it, when I add a product to the cart I get two
   > error messages at the top of the page:
   >  Warning: call_user_func_array() expects
   > parameter 1 to be a valid callback, function ‘__return_null()’ not found or
   > invalid function name in /home/customer/www/perlighting.com/public_html/wp-
   > includes/class-wp-hook.php on line 289
   > Warning: Cannot modify header information – headers already sent by (output
   > started at /home/customer/www/perlighting.com/public_html/wp-includes/class-
   > wp-hook.php:289) in /home/customer/www/perlighting.com/public_html/wp-content/
   > themes/generatepress/header.php on line 81
   > The question is I have removed the line from the functions.php and the first
   > message has disappeared, but now I get another message different from the previous
   > ones that I don’t know how to fix and is the following:
   >  Warning: Cannot modify
   > header information – headers already sent by (output started at /home/customer/
   > www/perlighting.com/public_html/wp-content/themes/generatepress/header.php:
   > 12) in /home/customer/www/perlighting.com/public_html/wp-content/themes/generatepress/
   > header.php on line 81
   > I would appreciate any help as I’m going crazy.
 * When you call `add_filter`, you pass the name of the function to call. You are
   including the `()` as part of the name, so that doesn’t work. Remove the space
   and the `()` to fix that.
    You mention the “headers already sent” warning from
   your theme twice. Any time you have a PHP warning or error that is displayed,
   you will get the “headers already sent” message. You shouldn’t be running a production
   site with Debug message on, though. See [https://wordpress.org/support/article/editing-wp-config-php/#wp_debug](https://wordpress.org/support/article/editing-wp-config-php/#wp_debug)
   to see how to turn it off or direct the messages to a log file.
 *  Thread Starter [periago89](https://wordpress.org/support/users/periago89/)
 * (@periago89)
 * [5 years, 8 months ago](https://wordpress.org/support/topic/error-warning-cannot-modify-header-information-8/#post-13516525)
 * Buenos días, el primer problema se ha solucionado, muchísimas gracias.
 * El segundo problema persiste y me sigue dando el siguiente mensaje:
 * Warning: Cannot modify header information – headers already sent by (output started
   at /home/customer/www/perlighting.com/public_html/wp-content/themes/generatepress/
   header.php:12) in /home/customer/www/perlighting.com/public_html/wp-content/themes/
   generatepress/header.php on line 81
 * No consigo solucionarlo, ya que es la primera web que hago y no manejo muy bien
   los temas de código.
 * ¿Alguna solución mas fácil?
 *  [Joy](https://wordpress.org/support/users/joyously/)
 * (@joyously)
 * [5 years, 8 months ago](https://wordpress.org/support/topic/error-warning-cannot-modify-header-information-8/#post-13517406)
 * > Good morning, the first problem has been solved, thank you very much.
   >  The 
   > second problem persists and keeps giving me the following message: Warning:
   > Cannot modify header information – headers already sent by (output started 
   > at /home/customer/www/perlighting.com/public_html/wp-content/themes/generatepress/
   > header.php:12) in /home/customer/www/perlighting.com/public_html/wp-content/
   > themes/generatepress/header.php on line 81
   > I can’t fix it, since It is the first website that I do and I do not handle
   > code issues very well.
   >  Any easier solution?
 * Did you change something in the theme header? Even a space that is outside the`
   <?php ?>` tags is considered output, and when you start output, the headers are
   sent. Then when WordPress tries to send the correct headers before loading the
   template, this warning is shown (because the headers were already sent accidentally).
   Again, you should be turning off the messaging so this problem doesn’t occur.
   
   If you didn’t change the theme header file, you can switch to a different theme
   to see if it’s still a problem. If it isn’t, you should ask at the theme’s support
   forum for what changed. You can use the [Health Check](https://wordpress.org/plugins/health-check/)
   plugin in Troubleshoot mode, to disable plugins and theme **for your user only**,
   to help you isolate the problem code.

Viewing 3 replies - 1 through 3 (of 3 total)

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## Tags

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

 * In: [Fixing WordPress](https://wordpress.org/support/forum/how-to-and-troubleshooting/)
 * 3 replies
 * 2 participants
 * Last reply from: [Joy](https://wordpress.org/support/users/joyously/)
 * Last activity: [5 years, 8 months ago](https://wordpress.org/support/topic/error-warning-cannot-modify-header-information-8/#post-13517406)
 * Status: not resolved

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