PHP:
Writing to standard error

How to:

In PHP, writing to stderr can be achieved using the fwrite() function alongside the predefined constant STDERR, which represents the error output stream.

<?php
// Writing a simple message to stderr.
fwrite(STDERR, "This is an error message.\n");

Sample output when the script is executed from the command line:

This is an error message.

To demonstrate more practical usage, consider a scenario where you’re parsing user input and encounter unexpected data:

<?php
$input = 'unexpected data';

// Simulating an error processing user input.
if ($input === 'unexpected data') {
    fwrite(STDERR, "Error: Unexpected input received.\n");
    exit(1); // Exiting with a non-zero value to indicate an error.
}

While PHP’s built-in capabilities for handling stderr are generally sufficient, when dealing with more complex applications or wanting to integrate stderr logging with external systems, third-party libraries like Monolog can be a powerful ally. Monolog is a logging library that can handle stderr among many other targets (files, sockets, etc.).

Using Monolog to write to stderr:

First, ensure you have Monolog installed via Composer:

composer require monolog/monolog

Then, you can configure Monolog to use the StreamHandler targeted at php://stderr:

<?php
require 'vendor/autoload.php';

use Monolog\Logger;
use Monolog\Handler\StreamHandler;

// Create a log channel
$log = new Logger('name');
$log->pushHandler(new StreamHandler('php://stderr', Logger::WARNING));

// Add a log message to stderr
$log->warning('This is a warning message.');

The above code utilizes Monolog to send a warning message to stderr, which is particularly useful for applications that require detailed logging configurations or external log monitoring.