Fixing “Headers Already Sent” Error in PHP

The “Headers already sent” error is one of the most common issues PHP developers face, especially when working with session_start(), setcookie(), or header() functions. This error can break functionality like user authentication, redirections, and cookie-based sessions.

In this article, we’ll explore the causes of this issue, how to debug it, and the best ways to prevent it from happening in your PHP applications.

Detailed Problem Explanation

The error message usually looks like this:

Warning: Cannot modify header information - headers already sent by (output started at /path/to/script.php:10) in /path/to/script.php on line 20

Why Does This Error Occur?

This error occurs because PHP sends HTTP headers before any output (like HTML or echo statements). However, if any output (even a blank space or newline) is sent before calling header() or session_start(), PHP throws this warning.

Common Causes:

• Whitespace or newlines before <?php or after ?>Example of a problematic file:

  ⬜<?php  
  session_start();  
  

• Echo, print, or HTML output before header functionsExample:

  echo "Welcome!";
  header("Location: dashboard.php"); // This will trigger an error
  

• Closing ?> tag at the end of PHP filesSome developers add an extra newline after ?>, causing unwanted output.
• Including files with unintended whitespaceIf an included file (include or require) has spaces or newlines, it can cause this error.
• Byte Order Mark (BOM) in UTF-8 filesIf a PHP file is saved with BOM, invisible characters are sent before headers.

Step-by-Step Solution

Step 1: Enable Error Reporting

Before fixing the issue, enable error reporting in your PHP script to see where the issue is coming from:

error_reporting(E_ALL);
ini_set('display_errors', 1);

This will help pinpoint which file and line are causing the issue.

Step 2: Remove Any Unintentional Output

Check for extra spaces before <?php and after ?>

Open all included PHP files and remove any blank lines before <?php.

Example of a correct PHP file:

<?php
session_start();

If ?> is at the end of a file, remove it entirely:

<?php
echo "Hello, World!";
// No closing tag here

Ensure No Output Before header() or session_start()

Move all output-related functions after headers:

<?php
session_start();
header("Location: dashboard.php");
exit; // Always use exit() after header redirection

Incorrect example (causes error):

echo "Redirecting...";
header("Location: dashboard.php");

Step 3: Check Included Files

If you’re using include or require, check that those files don’t have spaces or unexpected output.

Example:

include 'config.php'; // Ensure config.php has no unwanted output
session_start();

Use Output Buffering as a Quick Fix

If you can’t find the issue, temporarily use ob_start() at the beginning of your script:

<?php
ob_start();
session_start();
header("Location: dashboard.php");
ob_end_flush();
exit;

This buffers any accidental output, but it’s better to fix the root cause.

Step 4: Check for BOM Issues

If you still can’t find the problem, check if your file has a Byte Order Mark (BOM):

1. Open the file in a text editor like Notepad++
2. Go to Encoding > Convert to UTF-8 without BOM
3. Save the file and re-upload it

Additional Tips & Variations

Use headers_sent() to Debug

If you suspect headers have already been sent, check before calling header():

if (!headers_sent()) {
    header("Location: dashboard.php");
    exit;
} else {
    echo "Headers already sent.";
}

Check the php.ini File

If your server has output compression enabled (output_buffering = Off), consider enabling it:

output_buffering = 4096

Use a Code Editor with Whitespace Highlighting

Editors like VS Code, Sublime Text, or PHPStorm highlight spaces and BOM characters.

Conclusion

The “Headers already sent” error occurs when output is sent before HTTP headers. By ensuring no unwanted whitespace, using output buffering wisely, and verifying file encoding, you can easily prevent and fix this issue.

By following these debugging techniques, you’ll avoid frustration and keep your PHP scripts error-free. 🚀 Happy coding!