core-bundle/src/Resources/contao/library/Contao/System.php
public static function log($strText, $strFunction, $strCategory)
	{
		@trigger_error('Using System::log() has been deprecated and will no longer work in Contao 5.0. Use the logger service instead.', E_USER_DEPRECATED);

		$level = 'ERROR' === $strCategory ? LogLevel::ERROR : LogLevel::INFO;
		$logger = static::getContainer()->get('monolog.logger.contao');

		$logger->log($level, $strText, array('contao' => new ContaoContext($strFunction, $strCategory)));
	}
public static function log($strText, $strFunction, $strCategory)
	{
		@trigger_error('Using System::log() has been deprecated and will no longer work in Contao 5.0. Use the logger service instead.', E_USER_DEPRECATED);

		$level = 'ERROR' === $strCategory ? LogLevel::ERROR : LogLevel::INFO;
		$logger = static::getContainer()->get('monolog.logger.contao');

		$logger->log($level, $strText, array('contao' => new ContaoContext($strFunction, $strCategory)));
	}
Add a log entry to the database

@param string $strText     The log message
@param string $strFunction The function name
@param string $strCategory The category name

@deprecated Deprecated since Contao 4.2, to be removed in Contao 5.
Use the logger service instead.