How to Fix the “Headers Already Sent” Error in PHP

 The “Cannot modify header information – headers already sent” error is a common PHP issue that can also appear in WordPress development. It usually occurs when PHP tries to send or modify HTTP headers after some output has already been sent to the browser.

For example, functions such as header(), session_start(), and setcookie() need to run before PHP outputs anything.

Why Does This Error Happen?

A typical mistake looks like this:

echo "Welcome";

header("Location: dashboard.php");

Once echo sends output, PHP can no longer modify the HTTP headers. The correct approach is to handle the header first:

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

But visible output isn't the only cause.

Common Reasons for This Error

• echo, print, or debugging functions running before header()
• HTML appearing before PHP sends headers
• Extra spaces or blank lines before <?php
• Whitespace after the closing ?> tag
• A previous PHP warning or notice being displayed
• A UTF-8 BOM or other invisible characters in the file

How Can You Find the Real Problem?

Read the complete error message carefully. It usually tells you both where the header() call failed and where the output originally started.

For example:

headers already sent by (output started at file.php:25)

The most useful part is usually file.php:25. Check that location first for unexpected output, whitespace, HTML, or debugging code.

A Simple PHP Development Tip

Keep your request processing separate from page output. Run redirects, sessions, cookies, and other header-related logic before any HTML is sent.

Also, if a PHP file contains only PHP code, consider omitting the closing ?> tag. This helps prevent accidental whitespace from being added after the file.

The error may look complicated, but the main rule is simple:

Send or modify headers first. Send page output afterward.

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