\n用于Unix系统(包括Linux和OSX).

\n is used for Unix systems (including Linux, and OSX).

\r\n主要在Windows上使用.

\r\n is mainly used on Windows.

\r用于真正的旧Mac.

\r is used on really old Macs.

PHP_EOL常量代替这些字符，以实现平台之间的可移植性.

PHP_EOL constant is used instead of these characters for portability between platforms.