Description:
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The manual says this on sorting:

"If two members compare as equal, their order in the sorted array is undefined. Up to PHP 4.0.6 the user defined functions would keep the original order for those elements, but with the new sort algorithm introduced with 4.1.0 this is no longer the case as there is no solution to do so in an efficient way."

Bug #18299 requests that the old ("stable") sort behavior be restored. This is denied on the grounds that there is no efficient implementation for that.

Now it *is* possible to stabilize any sort algorithm, by comparing key positions if values compare equal.

Um, well, I can imagine that it's inefficient to determine the position of a key. In that case, I'd propose to introduce a SORT_STABLE flag and have the engine create a key=>position array when sorting with SORT_STABLE.
That would still be more efficient than trying to stabilize the sort at the PHP level. There, I'd have to do the following to get a stable sort:
* Make a copy of the array,
  wrapping each value in an array ($position, $value)
* Sort with a user-defined function. Have that function compare
  $values, and if these compare equal, compare $positions.
* Unwrap the $values from the resulting sorted array.
In effect, this means the engine will have to construct and garbage collect an array for each array element, so this is horribly inefficient compared to any in-engine solution...