PHP PSRs : PSR-3 Logger Interface

PSR-3 (PHP Standard Recommendation 3) defines a common interface for logging libraries to ensure interoperability between different logging implementations. It provides a standardized way for frameworks, libraries, and applications to log messages with…


This content originally appeared on DEV Community and was authored by Antonio Silva

PSR-3 (PHP Standard Recommendation 3) defines a common interface for logging libraries to ensure interoperability between different logging implementations. It provides a standardized way for frameworks, libraries, and applications to log messages without being tightly coupled to a specific logging implementation.

Key Concepts of PSR-3

Psr\Log\LoggerInterface

  • Defines a standard set of logging methods.
  • Implementations must provide these methods to handle logging at different severity levels.

Log Levels (Psr\Log\LogLevel)

  • PSR-3 defines eight severity levels:
    1. emergency: System is unusable.
    2. alert: Action must be taken immediately.
    3. critical: Critical conditions (e.g., system crash).
    4. error: Runtime errors that require attention.
    5. warning: Exceptional situations but not errors.
    6. notice: Normal but significant events.
    7. info: General information.
    8. debug: Debugging messages.

log() Method

  • The interface provides a generic log($level, $message, array $context = []) method.
  • This allows logging messages dynamically without needing to call specific methods.

Context Array

  • The $context parameter is an associative array used to pass additional information.
  • Supports placeholders in messages, replacing {key} with values from $context.

LoggerInterface Definition 

namespace Psr\Log;

interface LoggerInterface {
    public function emergency(string|\Stringable $message, array $context = []): void;
    public function alert(string|\Stringable $message, array $context = []): void;
    public function critical(string|\Stringable $message, array $context = []): void;
    public function error(string|\Stringable $message, array $context = []): void;
    public function warning(string|\Stringable $message, array $context = []): void;
    public function notice(string|\Stringable $message, array $context = []): void;
    public function info(string|\Stringable $message, array $context = []): void;
    public function debug(string|\Stringable $message, array $context = []): void;
    public function log(string $level, string|\Stringable $message, array $context = []): void;
}

Example Implementations

Custom PSR-3 Logger Implementation
If you want to create your own logger that follows PSR-3, implement LoggerInterface:

use Psr\Log\LoggerInterface;
use Psr\Log\LogLevel;

class FileLogger implements LoggerInterface {
    private string $logFile;

    public function __construct(string $logFile) {
        $this->logFile = $logFile;
    }

    public function log(string $level, string|\Stringable $message, array $context = []): void {
        $contextString = json_encode($context);
        $entry = strtoupper($level) . ': ' . $message . ' ' . $contextString . PHP_EOL;
        file_put_contents($this->logFile, $entry, FILE_APPEND);
    }

    public function emergency(string|\Stringable $message, array $context = []): void { $this->log(LogLevel::EMERGENCY, $message, $context); }
    public function alert(string|\Stringable $message, array $context = []): void { $this->log(LogLevel::ALERT, $message, $context); }
    public function critical(string|\Stringable $message, array $context = []): void { $this->log(LogLevel::CRITICAL, $message, $context); }
    public function error(string|\Stringable $message, array $context = []): void { $this->log(LogLevel::ERROR, $message, $context); }
    public function warning(string|\Stringable $message, array $context = []): void { $this->log(LogLevel::WARNING, $message, $context); }
    public function notice(string|\Stringable $message, array $context = []): void { $this->log(LogLevel::NOTICE, $message, $context); }
    public function info(string|\Stringable $message, array $context = []): void { $this->log(LogLevel::INFO, $message, $context); }
    public function debug(string|\Stringable $message, array $context = []): void { $this->log(LogLevel::DEBUG, $message, $context); }
}

// Usage
$logger = new FileLogger(__DIR__ . '/app.log');
$logger->info('User logged in', ['user_id' => 123]);
$logger->error('Database connection failed');

Using Monolog (PSR-3 Compliant)

The most widely used PSR-3 logger in PHP is Monolog.

Installation

composer require monolog/monolog

Example Usage

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

// Create a logger instance
$log = new Logger('my_logger');

// Add a handler (logs to a file)
$log->pushHandler(new StreamHandler(__DIR__ . '/app.log', Logger::DEBUG));

// Log messages at different levels
$log->info('This is an info message');
$log->warning('This is a warning');
$log->error('An error occurred', ['exception' => 'Database connection failed']);

Using PSR-3 in a Framework

Most PHP frameworks (Laravel, Symfony, etc.) use PSR-3-compatible logging systems.

Example: PSR-3 in Laravel

Laravel’s logger (based on Monolog) is PSR-3 compliant:

use Illuminate\Support\Facades\Log;

Log::info('User logged in', ['user_id' => 123]);
Log::error('Payment failed', ['error' => 'Insufficient funds']);

Conclusion

  • PSR-3 provides a standardized way to log messages across PHP applications.
  • Monolog is the most popular PSR-3-compliant library.
  • You can implement a custom logger by implementing LoggerInterface.
  • Frameworks like Laravel and Symfony already support PSR-3.


This content originally appeared on DEV Community and was authored by Antonio Silva


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