SPL is a collection of interfaces and classes that are meant to solve standard problems.
A more detailed documentation of SPL can be found » here.
This extension is available and compiled by default in PHP 5.
以下常量由本扩展模块定义，因此只有在本扩展模块被编译到 PHP 中，或者在运行时被动态加载后才有效。
SPL uses class constants since PHP 5.1. Prior releases use global constants in the form RIT_LEAVES_ONLY.