* get_parent_class() argument is optional
* Mark array_filter() $callback as optional
* The $base of gmp_strval() is optional
* DateTime constructor also accepts zero arguments
* hash_update_file() stream context is optional
* xmlwriter_write_dtd_entity() $isparam argument is optional
Signature stubs for internal functions are specified in xyz.stub.php,
from which we generate actual arginfo structures in xyz_arginfo.h.
This file then needs to be included in the implementation appropriately.
Arginfo from stubs can be regenerated using scripts/dev/gen_stub.php.
However, this should also automatically happen when the stub file is
modified.
