Description:
------------
I updated a library which did use \ArrayObject and access its key with the key function. I tested the script with both 7.4.0 and current 7.4.2 the result of the `key()` call on \ArrayObject does return in 7.4 always null instead on 7.3 it returns successfully the key. For me it looks like all array functions has no effect on the \ArrayObject.

Test script:
---------------
// should return `string(4) "test"` will return instead `NULL`
var_dump(
   key(
       new \ArrayObject(["test" => "123"])
   )
);