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PHP New Added 9 October 2026

PHP: Warning: Cannot modify header information - headers already sent by

Something was written to the response before header(), setcookie() or session_start() ran, and once the first byte has gone out the headers are fixed. The message names the file and line where output started, and the output is often invisible: whitespace after a closing ?>, a UTF-8 byte order mark, or a notice printed because display_errors is on. That last one is why it can happen on one server only.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix
# The warning says where: "(output started at /var/www/app/config.php:12)"

# A byte order mark before <?php shows up as ef bb bf
head -c 3 config.php | xxd
sed -i '1s/^\xEF\xBB\xBF//' config.php

# Whitespace after a closing tag at the end of a file
grep -rlzP '\?>\s+\z' src/
# In pure PHP files, leave out the closing ?> altogether

# Notices printed into the page: log them instead in production
php -i | grep display_errors
# php.ini: display_errors = Off, log_errors = On

How to diagnose PHP errors

PHP errors are dominated by per-request resource limits (memory_limit, max_execution_time) and by autoloading. Class-not-found errors are almost never missing code. They are a namespace that does not match the PSR-4 mapping, or a stale Composer autoload map. Laravel adds a caching layer that produces genuinely confusing errors when configuration is cached with the wrong environment.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

  1. Read the real limits from the running process, not from a config file: php -i | grep -E 'memory_limit|max_execution_time'. CLI and FPM have separate configurations.
  2. Regenerate the autoload map with composer dump-autoload -o before investigating a class-not-found error any further.
  3. For Laravel, clear cached config and routes with php artisan optimize:clear. A config cache built in the wrong environment causes errors that survive every code change.
  4. Enable full error display in development (display_errors=On, error_reporting=E_ALL). The default production settings hide the cause.
  5. For PDO connection refusals, test the same credentials with the database's own CLI client to separate PHP configuration from database access.

Tools worth reaching for

  • php -i
  • composer dump-autoload -o
  • php artisan optimize:clear
  • Xdebug
  • tail -f storage/logs/laravel.log

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

php.net php.net

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