Description:
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When SplDoublyLinkedList::setIteratorMode is called it overwrites its internal flag SPL_DLLIST_IT_FIX.

This allows mode changes for SplStack/SplQueue which otherwise would result in an exception.

This is different from #63175, as it is not related to user defined flags.

A proposed patch is attached.

Test script:
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<?php

$q = new SplQueue();
$q->setIteratorMode(SplDoublyLinkedList::IT_MODE_FIFO);
$q->setIteratorMode(SplDoublyLinkedList::IT_MODE_LIFO);

$r = new SplQueue();
$r->setIteratorMode(SplDoublyLinkedList::IT_MODE_LIFO);

Expected result:
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RuntimeException: Iterators' LIFO/FIFO modes for SplStack/SplQueue objects are frozen in test.php on line 5
RuntimeException: Iterators' LIFO/FIFO modes for SplStack/SplQueue objects are frozen in test.php on line 8

Actual result:
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RuntimeException: Iterators' LIFO/FIFO modes for SplStack/SplQueue objects are frozen in test.php on line 8