Arrays currently have a number of bizarre behaviors (especially in PHP) and limited support for manipulation makes this difficult to work around. The following additional functions would help:

$insert - atomic insertion into an array at an index.
Currently the only options are $push (which only works if you are adding to the end) and overwriting the entire array.

$remove - remove elements from an array BY INDEX, without leaving "null" entries.
Currently the only universally safe option is to overwrite the entire array. $unset leaves null entries, $pull works only if the element is known to be unique, and $unset followed by $pull null only works if the array is known to not contain nulls (also because $unset + $pull requires two separate operations, it is clunky and non-atomic (array contains null for a period of time)).