预定义常量
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。
Warning
SPL uses class constants since PHP 5.1. Prior releases use global constants
in the form RIT_LEAVES_ONLY.
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RecursiveIteratorIterator::LEAVES_ONLY
(integer)
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RecursiveIteratorIterator::SELF_FIRST
(integer)
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RecursiveIteratorIterator::CHILD_FIRST
(integer)
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CachingIterator::CALL_TOSTRING
(integer)
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CachingIterator::CATCH_GET_CHILD
(integer)
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