This is the F3 custom Logger class.
You can create a new Logger object and specify a log file like this:
$logger = new Log('error.log');
The path the Logger will use for log files is defined in the global F3 LOGS var. If the file and folder does not exist, the Logger tries to create them.
Write specified text to log file
$logger->write( string $text [, string $format = 'r' ] );
$format argument defines the date format that is going to be added to the log line. The default value
r specifies a RFC 2822 compliant date format that looks like
Thu, 21 Dec 2000 16:01:07 +0200. The remote address is also added to the log entry.
erases the whole log file