SPL - Standard PHP Library SPL is a collection of interfaces and classes that are meant to solve standard problems and implements some efficient data access interfaces and classes. You'll find the classes documented using php code in the file spl.php or in corresponding .inc files in subdirectories examples and internal. Based on the internal implementations or the files in the examples subdirectory there are also some .php files to experiment with.
The .inc files are not included automatically because they are sooner or later integrated into the extension. That means that you either need to put the code of examples/autoload.inc into your autoprepend file or that you have to point your ini setting auto_prepend_file to that file.
Below is a list of interfaces/classes already availabel natively through the SPL extension grouped by category.