How to Fix the “Cannot Modify Header Information – Headers Already Sent By” Error

A girl trying to modify header information on her computer

Most WordPress error messages give you an idea of what’s causing problems on your site. The “Warning: cannot modify header information – headers already sent by” error is no exception. If a PHP file cannot be executed due to a problem in its code, you’ll run into this message.

There are several potential causes for the “Cannot modify header information” error. Fortunately, the message itself will tell you which file is causing the problem. It even points to the line of code that contains the issue.

In this article, we’re going to discuss this error and its causes. Then, we’ll go over two ways that you can fix the problem. Let’s get to work!

What Causes the “Cannot Modify Header Information – Headers Already Sent By” Error

As we mentioned before, you’ll run into this error when one of your site’s .php files cannot be executed. WordPress relies on .php files, such as wp-config.php and functions.php, for its core functionality.

If there’s a problem within one of the .php files that your website needs to load, you’ll see an error message that looks like this:

Warning: Cannot modify header information - headers already sent by (output started at /home/public_html/wp-config.php:#) in /home/public_html/wp-includes/file-example.php on line 33

Fortunately, the “Cannot modify header information” error provides a lot of information that makes troubleshooting relatively straightforward. The message will point you toward two files – the first one contains the problem, which prevents the second one from executing.

At the end of the error message, you’ll see a section that says “line XX.” It shows the location of the specific code that’s causing the problem.

Usually, the problem in the PHP code is pretty easy to fix. Some common causes that can trigger the error message include:

  • Whitespaces before the <?phpsegment of the code or after the closing ?> tag
  • An HTML block before the PHP header function
  • print or echo statements added before the PHP header function
  • Problems with a plugin’s code

Fixing these types of errors requires you to be at least passingly comfortable with modifying PHP code. You won’t need to add any code yourself.

Still, you may need a bit of extra help identifying the problem. This is particularly true if the issue isn’t related to whitespaces or statements before the PHP header function.

The silver lining to seeing this error message- you already know which file is causing the problem and the line of code with the issue! ? Learn how to fix it here ?Click to Tweet

How To Troubleshoot the “Warning: Cannot Modify Header Information – Headers Already Sent By” Error (2 Methods)

There are two approaches to troubleshooting the



This article was written by Salman Ravoof and originally published on Blog – Kinsta®.

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