Returns an array with all keys from array lowercased or uppercased. Numbered indices are left as is.
The array to work on
Either CASE_UPPER or CASE_LOWER (default)
Returns an array with its keys lower or uppercased, or null if array is not an array.
Exceptions and Errors
Throws E_WARNING if array is not an array.
If an array has indices that will be the same once run through this function (e.g. "keY" and "kEY"), the value that is later in the array will override other indices.