Description:
------------
When using $index_key to index the resulting array by a column from the original array, if null values are encountered, the resulting keys become integer values that are auto-incremented according to the previous integer key value (or start from 0, as a 0-indexed key).

Test script with actual result and expected result is available here:
https://gist.github.com/ramsey/da33c07d95bfd58ea29a


Jan Roald Haugland first reported this issue on my blog:
http://benramsey.com/blog/2013/03/introducing-array-column-in-php-5-dot-5/#comment-1660885919

Test script:
---------------
See link to gist.

Expected result:
----------------
NULL keys should be converted to empty strings, and multiple matching keys should be overwritten by the last value entered for that key.

For example:

$foo = [
    null => 1,
    null => 2,
    null => 3
];

becomes

[
    '' => 3
]

Actual result:
--------------
With array_column(), NULL keys are being converted to 0-indexed integer key values, incrementing.

So an array that gets evaluated to this and, therefore, should result in the expected array shown above:

[
    null => 1,
    null => 2,
    null => 3
]

ends up resulting in this:

[
    0 => 1,
    1 => 2,
    2 => 3
]

See link to gist for full example and actual output.