Using Monolog for Logging

What is Monolog?

Monolog is a comprehensive logging library for PHP that sends your log records to files, sockets, inboxes, databases and various web services. It’s perfect for tracking application behavior, debugging issues, and monitoring your REST API built with Slim framework.

Key benefits:

PSR-3 compliant logging interface
Multiple handlers for different output destinations
Processors to add extra information to log records
Flexible formatting options
Support for log levels from DEBUG to EMERGENCY

Installation

Install Monolog using Composer:

composer require monolog/monolog

bash

However, Monolog is already included in the provided Slim template. So you can skip this step.

Using Monolog with Slim Framework

Import Monolog classes in your application

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

php

Create a logger instance with a channel name

// Create a logger with a channel name (e.g., 'app_access', 'api', 'database').
$logger = new Logger('app_access');

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Add one or more handlers to define where logs go

// Log to a file - INFO level and above.
$logger->pushHandler(new StreamHandler(__DIR__ . '/logs/app.log', Logger::INFO));

// You can add multiple handlers for different purposes.
// For example, log errors to a separate file.
$logger->pushHandler(new StreamHandler(__DIR__ . '/logs/errors.log', Logger::ERROR));

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Use the logger throughout your application

$app->post('/api/planets', function ($request, $response) use ($logger) {
    $data = $request->getParsedBody();

    // Log the incoming request.
    $logger->info('Creating new planet', ['data' => $data]);

    // Your business logic here...

    return $response->withStatus(201);
});

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Understanding Log Levels

Monolog implements the PSR-3 logging standard with eight severity levels. Use the appropriate level based on the type of information you’re logging:

Level	Method	When to Use	Example
DEBUG	`$logger->debug()`	Detailed debug information	Variable values, loop iterations
INFO	`$logger->info()`	Interesting events	User login, API requests
NOTICE	`$logger->notice()`	Normal but significant events	Configuration changes
WARNING	`$logger->warning()`	Exceptional occurrences that are not errors	Use of deprecated APIs, poor API usage
ERROR	`$logger->error()`	Runtime errors that don’t require immediate action	Database connection failed
CRITICAL	`$logger->critical()`	Critical conditions	Application component unavailable
ALERT	`$logger->alert()`	Action must be taken immediately	Entire website down
EMERGENCY	`$logger->emergency()`	System is unusable	Database unavailable, system crash

Common Logging Patterns

Logging Request Information
Logging Database Operations
Logging API Calls
Logging Exceptions
Logging Authentication Events

Adding Context to Logs

Monolog allows you to add contextual information to your log messages using arrays:

// Simple context.
$logger->info('User logged in', ['user_id' => 123]);

// Rich context.
$logger->error('Payment processing failed', [
    'user_id' => 123,
    'amount' => 99.99,
    'currency' => 'USD',
    'error_code' => 'CARD_DECLINED',
    'timestamp' => time()
]);

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Configuring Time Zone and Log Format

Setting the Time Zone

By default, Monolog uses UTC for timestamps. You can change this globally for all loggers:

use Monolog\Logger;

$logger = new Logger('access');
// Set the default timezone for this logger.
$logger->setTimezone(new \DateTimeZone('America/New_York'));

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Customizing Log Format

Monolog uses formatters to control how log records appear. The default LineFormatter can be customized:

use Monolog\Handler\StreamHandler;
use Monolog\Formatter\LineFormatter;

$handler = new StreamHandler(__DIR__ . '/logs/app.log', Logger::INFO);

// Create a custom format.
// Available placeholders: %datetime%, %channel%, %level_name%, %message%, %context%, %extra%
$dateFormat = "Y-m-d H:i:s";
$outputFormat = "[%datetime%] %channel%.%level_name%: %message% %context%\n";

$formatter = new LineFormatter($outputFormat, $dateFormat);
$handler->setFormatter($formatter);

$logger = new Logger('app');
$logger->pushHandler($handler);

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Example output:

[2025-11-10 14:30:45] app.INFO: User logged in {"user_id":123}
[2025-11-10 14:31:12] app.ERROR: Database connection failed {"error":"Connection timeout"}

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