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

Cannot Modify Header Information – Headers Already Sent By

Dealing with frustrating errors like “Cannot modify header information – headers already sent” can be quite annoying for developers. This error essentially means that you are trying to send header information after the output has already been sent to the browser. Thankfully, this is a common issue that can be easily resolved with a few tweaks to your code.

What Causes the “Headers Already Sent” Error

The “headers already sent” error occurs because of the way headers and output work in PHP. HTTP headers must be sent before any output to the browser. If any output is sent before calling header() then you will get this error. There are a few common causes:

  • Whitespace characters – Leading whitespace characters like spaces or newlines at the beginning of a file can trigger output before headers.
  • print, echo statements – Any Functions like print, echo, readfile, etc will output content to the browser and trigger the error if called before header().
  • Errors and warnings – PHP errors and warnings will also output content before headers.

So the key is looking for any output that could be sent before your scripts sends headers.

How to Fix “Headers Already Sent” Errors

Fixing this error just involves identifying and removing any output that gets sent before your header() calls. Here are some tips:

Remove Whitespace Characters

Carefully check your files for accidental whitespace at the beginning of files, before the opening <?php tag. Be sure to look for:

  • Spaces
  • Newlines
  • Tabs
  • Carriage returns

Many developers miss this seemingly insignificant whitespace which can trigger output and cause headers already sent errors.

Move Calls to header() Up

If you are calling header() after echo or other output statements, try moving it above any output. Get header() calls sent before any other output starts.

Separate PHP Code from HTML

If you are embedding PHP directly in HTML, consider moving your PHP code into separate files that are included. This prevents accidental whitespace and makes it easier to send headers first.

Resolve Errors and Warnings

Double-check for and resolve any PHP errors or warnings. These will send output which triggers the headers already sent error.

Buffer Output

For smaller scripts, you can buffer output which will prevent anything being sent prematurely.

// Buffer output


// Headers 

header(‘Content-type: text/plain’);

header(‘Content-disposition: attachment; filename=file.txt’);

// Output 

echo ‘Hello World!’;

// Flush buffer


Identify the Source of Unwanted Output

In complex scripts, it can be tricky to locate exactly where output is coming from. A few tips:

  • Check error logs – Your PHP error logs may show the file and line number where the output started.
  • Enable output buffering – Enable output buffering in php.ini temporarily. This will buffer all output and prevent the error, letting you isolate where the first output is coming from.
  • Comment out sections – Use comments to remove sections of code at a time until you narrow down the source of output.
  • Use debugging tools – Debugging tools like Xdebug can help trace the output.

With some careful debugging, you should be able to zero in on any output that needs removal or relocation.

Avoid Common Causes in Your Code

Learning from these errors can help you adopt coding best practices that prevent headers already sent errors in the future:

  • Be careful with whitespace characters at the start of files.
  • Call header() before any output code.
  • Don’t embed PHP in HTML; keep presentation code separate.
  • Resolve all PHP errors, warnings and notices as they output content.
  • Use output buffering for better control in complex scripts.

Adopting these habits will help avoid those pesky “cannot modify header information” errors!

Check Server Settings and Versions

If you have checked your code thoroughly, also look at potential server issues:

  • Output buffering – Ensure output_buffering is enabled in php.ini.
  • Server software – Some WAMP/LAMP stacks have known issues causing white space errors. Updating them can help.
  • PHP versions – Switching PHP versions has resolved the issue for some users. Test your code on multiple versions.

Tightening up server settings and software versions can eliminate whitespace and output issues leading to these errors.

Use a Debugging Extension

Browser extensions like PHP Console can help debug headers already sent errors by showing you:

  • All headers set by a script
  • The order headers were sent
  • Details on any output before headers

Seeing this debugging info makes it much faster to identify issues in your code.

Troubleshooting with var_dump() and Commenting

For simple scripts, some basic debugging techniques can help:

  • Var_dump – Insert var_dump() and print specific variables at points in your code to see the order of execution and output.
  • Commenting – Temporarily comment out chunks of code to isolate the source of output.

Combined with checking error logs these simple tricks can quickly reveal the source of the headers already sent error.

Fixing Headers Sent Errors with a PHP Framework

If reorganizing procedural code is too complex, consider switching to an MVC framework like Laravel or Symfony. This enforces a separation between logic and display code, making it almost impossible to encounter these errors.

Frameworks handle all header and output buffering automatically, letting you focus on building your application services and UI without worrying about header sent order.

Common Confusions Around Headers Already Sent

Some developers assume that any header modifications after output cause this error. That is not quite accurate:

  • You can send as many additional headers as you want even after output.
  • Only trying to overwrite existing headers will trigger the error after output.

So don’t be afraid to continue setting headers after output when needed. Just avoid overwriting existing headers and you can avoid these tricky errors.

When to Ignore “Headers Already Sent” Errors

In some cases, you may need to just ignore or work around the headers already sent errors:

  • On high-traffic sites, output before headers is triggered by other scripts. Fixes may not be feasible.
  • During development, errors from included files can be ignored until you are ready to refactor code.
  • For simple scripts, buffering output as shown above lets you work around rather than fix the code issue.

While not ideal, temporarily ignoring the error is sometimes the pragmatic solution when dealing with legacy systems and complex sites.

Preventing Future Errors with Coding Best Practices

Learning to avoid these header errors will make you a better developer:

  • Always start PHP files with <?php – no whitespace before.
  • Call header() before echo or other output statements.
  • Use functions like require(), include() and loadFile() at the end of scripts.
  • Enable output buffering in php.ini if needed.
  • Separate application logic from presentation code.

Adopting these simple practices will help you avoid hours of frustration debugging header errors down the road!


The “cannot modify header information – headers already sent by” error can be tricky to resolve, but just requires some careful debugging and tweaking of your code. 

Checking for whitespace, reorganizing header and output statements, and isolating the source of premature output will usually fix the problem. Separating logic and display code also helps avoid these errors. With some careful error tracking and refactoring of your code, you can eliminate frustrating header sent errors.


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