"Cannot modify header information" error encountered while streaming #60603
Description
Symfony version(s) affected
7.3.0
Description
Hi, I'm trying to use Livewire streams in my Laravel app, but it keep on triggering "Cannot modify header information" error such as livewire/livewire#9357
After digging deeper into the codes, I found this PR #60377 that triggered above issue.
How to reproduce
You could check out my repo below. I installed a fresh new Laravel app with just Livewire installed to make sure nothing else was affecting the test results.
- Execute below commands
git clone git@github.com:sarukomine/livewire-stream-for-report.git
cd livewire-stream-for-report
git checkout 12.x
composer install
cp .env.example .env
php artisan key:generate
php artisan migrate
php artisan serve-
Open
http://127.0.0.1:8000/streampage -
Click "Start count-down" button
-
Open
storage/logs/laravel.logfile, you could see the error message
laravel/laravel => v10.3.3
laravel/framework => v10.48.29
livewire/livewire => v3.6.3
symfony/http-foundation => v6.4.22
laravel/laravel => v11.6.1
laravel/framework => v11.0.0
livewire/livewire => v3.6.3
symfony/http-foundation => v7.3.0
laravel/laravel => v12.0.9
laravel/framework => v12.16.0
livewire/livewire => v3.6.3
symfony/http-foundation => v7.3.0
Possible Solution
Just add the fix before trigger header() method, stream response will be works fine.
if (!empty(preg_grep('/^Content-Type:.*\btext\/event-stream\b.*/i', headers_list()))) {
return $this;
}like that
// headers have already been sent by the developer
if (headers_sent()) {
// Add a fix here
if (!empty(preg_grep('/^Content-Type:.*\btext\/event-stream\b.*/i', headers_list()))) {
return $this;
}
if (!\in_array(\PHP_SAPI, ['cli', 'phpdbg', 'embed'], true)) {
// Or add a fix here
if (!empty(preg_grep('/^Content-Type:.*\btext\/event-stream\b.*/i', headers_list()))) {
return $this;
}
$statusCode ??= $this->statusCode;
header(\sprintf('HTTP/%s %s %s', $this->version, $statusCode, $this->statusText), true, $statusCode);
}
return $this;
}Or, we may delete these changes from #60377, let this function keep simple~
Additional Context
[2025-05-30 13:11:31] local.ERROR: Cannot modify header information - headers already sent by (output started at /Users/whoami/Herd/livewire-stream-11/vendor/livewire/livewire/src/Features/SupportStreaming/SupportStreaming.php:37) {"exception":"[object] (ErrorException(code: 0): Cannot modify header information - headers already sent by (output started at /Users/whoami/Herd/livewire-stream-11/vendor/livewire/livewire/src/Features/SupportStreaming/SupportStreaming.php:37) at /Users/whoami/Herd/livewire-stream-11/vendor/symfony/http-foundation/Response.php:322)
[stacktrace]
#0 /Users/whoami/Herd/livewire-stream-11/vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(256): Illuminate\\Foundation\\Bootstrap\\HandleExceptions->handleError(2, 'Cannot modify h...', '/Users/whoami...', 322)
#1 [internal function]: Illuminate\\Foundation\\Bootstrap\\HandleExceptions->Illuminate\\Foundation\\Bootstrap\\{closure}(2, 'Cannot modify h...', '/Users/whoami...', 322)
#2 /Users/whoami/Herd/livewire-stream-11/vendor/symfony/http-foundation/Response.php(322): header('HTTP/1.1 200 OK', true, 200)
#3 /Users/whoami/Herd/livewire-stream-11/vendor/symfony/http-foundation/Response.php(401): Symfony\\Component\\HttpFoundation\\Response->sendHeaders()
#4 /Users/whoami/Herd/livewire-stream-11/vendor/laravel/framework/src/Illuminate/Foundation/Application.php(1168): Symfony\\Component\\HttpFoundation\\Response->send()
#5 /Users/whoami/Herd/livewire-stream-11/public/index.php(17): Illuminate\\Foundation\\Application->handleRequest(Object(Illuminate\\Http\\Request))
#6 /Applications/Herd.app/Contents/Resources/valet/server.php(167): require('/Users/whoami...')
#7 {main}
"}
[2025-05-30 13:11:31] local.ERROR: Uncaught ErrorException: Cannot modify header information - headers already sent by (output started at /Users/whoami/Herd/livewire-stream-11/vendor/livewire/livewire/src/Features/SupportStreaming/SupportStreaming.php:37) in /Users/whoami/Herd/livewire-stream-11/vendor/symfony/http-foundation/Response.php:322
Stack trace:
#0 /Users/whoami/Herd/livewire-stream-11/vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(256): Illuminate\Foundation\Bootstrap\HandleExceptions->handleError(2, 'Cannot modify h...', '/Users/whoami...', 322)
#1 [internal function]: Illuminate\Foundation\Bootstrap\HandleExceptions->Illuminate\Foundation\Bootstrap\{closure}(2, 'Cannot modify h...', '/Users/whoami...', 322)
#2 /Users/whoami/Herd/livewire-stream-11/vendor/symfony/http-foundation/Response.php(322): header('HTTP/1.1 500 In...', true, 500)
#3 /Users/whoami/Herd/livewire-stream-11/vendor/symfony/http-foundation/Response.php(401): Symfony\Component\HttpFoundation\Response->sendHeaders()
#4 /Users/whoami/Herd/livewire-stream-11/vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(219): Symfony\Component\HttpFoundation\Response->send()
#5 /Users/whoami/Herd/livewire-stream-11/vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(196): Illuminate\Foundation\Bootstrap\HandleExceptions->renderHttpResponse(Object(ErrorException))
#6 /Users/whoami/Herd/livewire-stream-11/vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(256): Illuminate\Foundation\Bootstrap\HandleExceptions->handleException(Object(ErrorException))
#7 [internal function]: Illuminate\Foundation\Bootstrap\HandleExceptions->Illuminate\Foundation\Bootstrap\{closure}(Object(ErrorException))
#8 {main}
thrown {"exception":"[object] (Symfony\\Component\\ErrorHandler\\Error\\FatalError(code: 0): Uncaught ErrorException: Cannot modify header information - headers already sent by (output started at /Users/whoami/Herd/livewire-stream-11/vendor/livewire/livewire/src/Features/SupportStreaming/SupportStreaming.php:37) in /Users/whoami/Herd/livewire-stream-11/vendor/symfony/http-foundation/Response.php:322
Stack trace:
#0 /Users/whoami/Herd/livewire-stream-11/vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(256): Illuminate\\Foundation\\Bootstrap\\HandleExceptions->handleError(2, 'Cannot modify h...', '/Users/whoami...', 322)
#1 [internal function]: Illuminate\\Foundation\\Bootstrap\\HandleExceptions->Illuminate\\Foundation\\Bootstrap\\{closure}(2, 'Cannot modify h...', '/Users/whoami...', 322)
#2 /Users/whoami/Herd/livewire-stream-11/vendor/symfony/http-foundation/Response.php(322): header('HTTP/1.1 500 In...', true, 500)
#3 /Users/whoami/Herd/livewire-stream-11/vendor/symfony/http-foundation/Response.php(401): Symfony\\Component\\HttpFoundation\\Response->sendHeaders()
#4 /Users/whoami/Herd/livewire-stream-11/vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(219): Symfony\\Component\\HttpFoundation\\Response->send()
#5 /Users/whoami/Herd/livewire-stream-11/vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(196): Illuminate\\Foundation\\Bootstrap\\HandleExceptions->renderHttpResponse(Object(ErrorException))
#6 /Users/whoami/Herd/livewire-stream-11/vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php(256): Illuminate\\Foundation\\Bootstrap\\HandleExceptions->handleException(Object(ErrorException))
#7 [internal function]: Illuminate\\Foundation\\Bootstrap\\HandleExceptions->Illuminate\\Foundation\\Bootstrap\\{closure}(Object(ErrorException))
#8 {main}
thrown at /Users/whoami/Herd/livewire-stream-11/vendor/symfony/http-foundation/Response.php:322)
[stacktrace]
#0 {main}
"}