Description:
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Attempting to change the IteratorMode throws an exception.

The is probably due to the code that changes the LIFO/FIFO ordering that is also settable via the inherited method of SplDoublyLinkedList::setIteratorMode().



Reproduce code:
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<?php

$s = new SPLStack(); 
$s->setIteratorMode(SplDoublyLinkedList::IT_MODE_DELETE);

Expected result:
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Do not throw exception on the following use cases:

->setIteratorMode(SplDoublyLinkedList::IT_MODE_DELETE)
->setIteratorMode(SplDoublyLinkedList::IT_MODE_KEEP)

Actual result:
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PHP Fatal error:  Uncaught exception 'RuntimeException' with message 'Iterators' LIFO/FIFO modes for SplStack/SplQueue objects are frozen' in path/to/test-splstack.php:4
Stack trace:
#0 path/to/test-splstack.php(4): SplDoublyLinkedList->setIteratorMode(1)
#1 {main}
  thrown in path/to/test-splstack.php on line 4