The Standard PHP Library (SPL) is a collection of interfaces and classes that are meant to solve common problems.
No external libraries are needed to build this extension and it is available and compiled by default in PHP 5.0.0.
SPL provides a set of standard datastructure, a set of iterators to traverse over objects, a set of interfaces, a set of standard Exceptions, a number of classes to work with files and it provides a set of functions like spl_autoload_register()