Mage::log('This is a log message', null, 'my_log_file.log');
Mage::log('This is a log message', null, 'system');
Mage::log('This is a debug message', Zend_Log::DEBUG);This code will write the string "This is a debug message" to the default log file with a log level of DEBUG. The log level can be set to any of the constants defined in the Zend_Log class. The package library for php Mage log would likely be the Magento core library, as it is a class that is included in the default installation of Magento.