protected function _initLogger()
 {
     require_once APPLICATION_PATH . '/helper/logger/logger.php';
     require_once APPLICATION_PATH . '/helper/logger/abstractStrategy.php';
     require_once APPLICATION_PATH . '/helper/logger/fileStrategy.php';
     $fileStrategy = new fileStrategy();
     $fileStrategy->setFilename(dirname(__FILE__) . '/../app_log/log_' . date('Y-m-d') . '.txt');
     logger::getInstance()->addLogStrategy($fileStrategy);
     logger::getInstance()->setLogLevel(logger::LEVEL_OK);
 }
<?php

require_once 'logger.php';
require_once 'abstractStrategy.php';
require_once 'printStrategy.php';
require_once 'fileStrategy.php';
require_once 'firePhpStrategy.php';
logger::getInstance()->addLogStrategy(new printStrategy());
logger::getInstance()->addLogStrategy(new firePhpStrategy());
$fileStrategy = new fileStrategy();
$fileStrategy->setFilename('log.txt');
logger::getInstance()->addLogStrategy($fileStrategy);
logger::getInstance()->log('Logeintrag xyz', logger::LEVEL_ERROR, "import");
logger::getInstance()->clearLogStrategy();