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<?php
// Defines a factory for logging channels.

/** @var \Drupal\Core\Logger\LoggerChannelFactory $logger */
$logger = \Drupal::service('logger.factory');
$logger_channel = $logger->get('module_name');
// notice, warning, info
$logger_channel->error('Error @error!', ['@error' => $error]);
// Log the object:
\Drupal::logger('module_name')->warning('<pre><code>' . print_r($obj, TRUE) . '</code></pre>');