Description:
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SplFixedArray thankfully throws exceptions when you try to do incorrect things 
with indices.  However, the types of exceptions are just too generic.  If I give 
the wrong type of index, that's a logic error (I'd expect InvalidArgument or at 
least something that inherits from LogicError to be thrown).  If I give an index 
that's that's a valid type but doesn't exist, I'd expect an OutOfBoundsException 
to be thrown.  Instead I get a generic RuntimeException.

I should expect because they are very different problems that I would at least 
get a distinguishing message between the two.  However, I get the same 
descriptions:  'Index invalid or out of range'.  The very message suggests they 
should be different exceptions.

The first fix would sort-of break backwards compatibility: throw an 
InvalidArgumentException for things of the wrong type.

The second fix, throw OutOfBoundsException on incorrect index, could be 
implemented and keep backwards compatibility.

Test script:
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$fa = new SplFixedArray(1);

$fa[new StdClass()]; //expect InvalidArgumentException or perhaps OutOfRangeException

$fa[2] = 'james'; // expect OutOfBoundsException

Expected result:
----------------
I expect $fa[new StdClass] to throw an InvalidArgumentException, not a 
RuntimeException

I expect $fa['2'] to throw OutOfBoundsException not a RuntimeException.

Actual result:
--------------
Both throw RuntimeExceptions.