PHP headers_sent() Function: Everything You Need to Know

Every HTTP response is split into two parts: a block of headers (status code, Content-Type, cookies, redirects) followed by the response body. The catch is that headers must be sent before any body output. The moment PHP echoes a single byte of body, it flushes the headers and locks them — any later call to header(), setcookie(), or session_start() then triggers the infamous Cannot modify header information - headers already sent warning.

headers_sent() is the built-in that lets you ask, before you try to send a header, whether it is already too late. This guide covers its syntax, its by-reference parameters, what causes premature output, and how to fix it.

What headers_sent() Does

headers_sent() returns a boolean:

• true — headers have already been sent (output started). Any further header() / setcookie() call will fail.
• false — no output has started yet, so it is still safe to set headers.

Guarding header logic with headers_sent() lets you avoid the warning and degrade gracefully (for example, log the problem or fall back to a JavaScript redirect) instead of crashing the page.

Syntax

headers_sent(string &$filename = null, int &$line = null): bool

Both parameters are optional and passed by reference. When headers have been sent, PHP fills them with the location of the output that started it:

• $filename — the name of the source file where output first began.
• $line — the line number within that file.

This is what makes headers_sent() so useful for debugging: it points you straight at the culprit.

Basic Usage

Check the return value before sending a header:

<?php

if (!headers_sent()) {
    header('Location: /dashboard');
    exit;
}

echo 'Headers were already sent, cannot redirect via HTTP.';

If output has not started, the redirect header is sent. Otherwise the script avoids the fatal warning and prints a fallback message.

Finding Where Output Started

Pass the two reference variables to learn exactly what sent the headers — invaluable when a stray space or echo is buried in an included file:

<?php

if (headers_sent($file, $line)) {
    echo "Headers already sent in $file on line $line";
} else {
    setcookie('theme', 'dark');
    echo 'Cookie set successfully.';
}

If the condition is true, you get a message like Headers already sent in /var/www/header.php on line 12, telling you precisely where to look.

What Triggers "Headers Already Sent"

Anything that produces body output before your header call flushes the headers. Common causes:

• Whitespace or a blank line before <?php — even one space or newline counts as output.
• A newline after the closing ?> of an included file. (Best practice: omit the closing ?> in pure-PHP files entirely.)
• echo, print, printf, or var_dump running before the header.
• A UTF-8 file saved with a BOM (byte-order mark) — those invisible leading bytes are output.
• PHP warnings/notices printed to the page (when display_errors is on) before headers.

Fixing It with Output Buffering

If you cannot reorder your code so all headers come first, wrap the script in an output buffer. ob_start() holds the body in memory instead of sending it immediately, so headers stay modifiable until the buffer is flushed:

<?php

ob_start();              // start buffering — nothing is sent yet

echo 'Some early output';

// Still safe: the echo above is held in the buffer, headers are not sent
setcookie('user', 'jane');
header('X-App-Version: 2.0');

ob_end_flush();          // now send headers, then the buffered body

Because output is buffered, headers_sent() would still return false after the echo, and the later setcookie() and header() calls succeed.

Related Functions

• header() — send a raw HTTP header.
• headers_list() — list headers queued or already sent.
• setcookie() — set a cookie (sends a Set-Cookie header).
• ob_start() — start output buffering to delay sending headers.
• PHP Sessions — session_start() also sends headers and is a frequent trigger.

Conclusion

headers_sent() is a small but essential guard: call it before any header(), setcookie(), or session_start() to check whether output has already begun. When it returns true, the by-reference $filename and $line arguments pinpoint the offending output so you can fix it — or wrap your script in ob_start() to keep headers modifiable until you are ready to flush.

Practice