The “Cannot modify header information – headers already sent by” error message is one of the most common errors experienced in PHP programming and WordPress. This error often occurs when PHP tries to send or change HTTP headers after the server has already started sending the response body to the client.

This error commonly occurs with redirects, session starts, cookie handling, or authentication. Any extra space, output function, or PHP error message can generate this error.

The problem faced by developers is determining the point at which the output was generated and knowing why PHP can no longer alter the response headers.

The good news is that the problem can easily be diagnosed once one understands how PHP manages the HTTP response.

This article explains the error message and how to fix it.

What Does “Cannot Modify Header Information – Headers Already Sent By” Mean?

The warning occurs when a PHP script attempts to modify HTTP headers after output has already been sent to the client.

HTTP responses contain two main components: headers and a body.

Headers provide information about the response, such as content type, cookies, caching instructions, and redirect locations. The response body contains the actual content, such as HTML, JSON, or text.

PHP normally sends headers before the response body. Once output has been transmitted, PHP cannot modify those headers.

Consider this example:

<?php
echo "Welcome to the website";
header("Location: dashboard.php");
exit;

The echo statement produces output before the redirect header is set.

If output has already been transmitted, PHP displays a warning similar to:

Warning: Cannot modify header information –

headers already sent by

(output started at /var/www/html/index.php:2)
in /var/www/html/index.php on line 4

The warning identifies where output started and where PHP attempted to modify the headers.

What Causes the Headers Already Sent Warning in PHP?

Several coding and configuration issues can trigger this warning. Most involve unexpected output before header-related operations.

<?php
echo "User authenticated";
header("Location: profile.php");
exit;

<?php
header("Location: profile.php");
exit;

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

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

<?php
echo $undefined_variable;
header("Location: home.php");
exit;

display_errors = Off
log_errors = On

How to Fix Cannot Modify Header Information – Headers Already Sent By?

The correct solution depends on where the output originates.

Warning: Cannot modify header information -
headers already sent by
(output started at /app/config.php:5)
in /app/login.php on line 20

echo "Debug message";
print "Testing";
var_dump($data);

<?php
require "config.php";
header("Location: dashboard.php");
exit;

<?php
if (!headers_sent($file, $line)) {
header("Location: dashboard.php");
exit;
}
error_log(
"Headers already sent in $file on line $line"
);

How to Fix Headers Already Sent During PHP Redirects?

Redirects are among the most common situations where this warning appears.

The header() function can send a Location response header that instructs the browser to navigate to another URL.

Incorrect:

<?php
echo "Login successful";
header("Location: dashboard.php");
exit;
Correct:
<?php
$is_authenticated = true;
if ($is_authenticated) {
header("Location: dashboard.php", true, 302);
exit;
}
echo "Authentication failed";

The redirect occurs before any response content is generated.

For login workflows, developers should also validate authentication and authorization before performing redirects.

Using exit after a redirect prevents unintended code execution.

How to Fix Headers Already Sent With PHP Sessions?

PHP sessions commonly require headers to be sent when creating or resuming a session.

If output occurs before session_start(), PHP may be unable to send the necessary session cookie.

Incorrect:

<?php
echo "Starting session";
session_start();
Correct:
<?php
session_start();
$_SESSION["username"] = "John";
echo "Session started";

Call session_start() before generating output whenever the application requires session initialization.

For larger applications, handle session management consistently during request initialization.

How to Fix Headers Already Sent When Setting Cookies?

PHP uses HTTP headers to communicate cookies to browsers.

The setcookie() function must execute before response headers are transmitted.

Incorrect:

<?php
echo "Welcome back";
setcookie("username", "John", time() + 3600);
Correct:
<?php setcookie( "username", "John", [ "expires" => time() + 3600,
"path" => "/",
"secure" => true,
"httponly" => true,
"samesite" => "Lax"
]
);
echo "Cookie configured";

The example assumes the application is served over HTTPS.

Cookie security settings should be selected according to the application’s authentication and cross-site requirements.

Can Output Buffering Fix Headers Already Sent?

PHP provides output buffering functions that temporarily store generated output before transmitting it.

For example:

<?php
ob_start();
echo "Processing request";
header("Location: dashboard.php");
exit;

Because the output remains buffered, the redirect header may still be modified.

However, output buffering should not be treated as the primary solution.

It can hide incorrect application flow, increase memory usage, and make debugging more difficult.

The preferred approach is to execute header-related operations before producing response content.

How to Prevent Headers Already Sent Errors in PHP

Preventing this warning requires consistent application structure and proper output management.

How Moon Technolabs Helps With PHP Development and Error Resolution

Moon Technolabs offers PHP development and web application maintenance to help companies build robust, secure, and scalable solutions. Our PHP developers can address PHP warnings, solve redirect- or session-related problems, enhance your application’s backend, and debug issues that may affect performance.

If you need to update your PHP application or resolve WordPress compatibility issues, Moon Technolabs helps you identify technical issues and implement sustainable solutions.

Conclusion

This PHP Warning, “Cannot modify header information – headers already sent by”, is thrown when PHP tries to modify HTTP headers even though some form of response has already been sent.

Several factors can cause this error, including premature echoes, extra spaces in PHP files, a UTF-8 BOM, PHP warnings visible to end users, and incorrect session or cookie ordering.

To solve the problem, examine the warning, locate the source of the output, and reposition the code that changes headers.

In WordPress, you can check theme and plugin changes and review the debug logs.

Developers should stick to coding standards to prevent these errors in the future.