spl_classes

The above example will output something similar to:

Array
(
    [ArrayObject] => ArrayObject
    [ArrayIterator] => ArrayIterator
    [CachingIterator] => CachingIterator
    [RecursiveCachingIterator] => RecursiveCachingIterator
    [DirectoryIterator] => DirectoryIterator
    [FilterIterator] => FilterIterator
    [LimitIterator] => LimitIterator
    [ParentIterator] => ParentIterator
    [RecursiveDirectoryIterator] => RecursiveDirectoryIterator
    [RecursiveIterator] => RecursiveIterator
    [RecursiveIteratorIterator] => RecursiveIteratorIterator
    [SeekableIterator] => SeekableIterator
    [SimpleXMLIterator] => SimpleXMLIterator
)