从cron运行时，脚本的工作目录可能不同。另外，关于PHPs require（）和include（）的理解有些混乱，这引起了关于工作目录确实是问题的困惑：
include('foo.php') // searches for foo.php in the same directory as the current scriptinclude('./foo.php') // searches for foo.php in the current working directoryinclude('foo/bar.php') // searches for foo/bar.php, relative to the directory of the current scriptinclude('../bar.php') // searches for bar.php, in the parent directory of the current working directory
