TYPO3 Core

Has been discussed for some other methods already in the core list, and adding type hinting is nice but breaks compatibility, because this is the API we used to have. Most common problem is:

$conf meant to be an array()

But extensions passing "NULL" to it. This is not compatible with type hinting "array" and will generate a fatal error.

So that cleanup can only be done by deprecating all methods and introducing new ones, or simply ignoring this backwards compatibility problem, which I would like to postpone to 4.6. :)

function test (array $arg = NULL) {

allows NULL, unset variables and array to be passed as argument. Other types trigger an exception.

You need to test for NULL and set $arg to an empty array and you're backwards compatible with NULL values.
At least passing a string will not cause erratic behaviour because PHP will otherwise trying to index strings.

Seems to be the same problem as #24462