[RFC]: Add file_descriptor() function to retrieve the file descriptor of a PHP Stream

[Girgias]

RFC: https://wiki.php.net/rfc/file-descriptor-function

The purpose of this function is to retrieve the underlying file descriptors for PHP streams when they exist.
It is currently possible to achieve this result by using FFI and stubbing the Zend engine (see: https://github.com/ppelisset/php-fileno).
This can be needed when trying to interact with a USB device, as this is the use case @ppelisset has.

Test should be based of #10173 when it gets merged.

[azjezz]

Suggested change

	zend_argument_type_error(1, "cannot represent as a file descriptor");
	zend_argument_type_error(1, "cannot be represented as a file descriptor");

[azjezz]

it think we miss a test case for closed resources, i.e:

$fp = fopen('somefile.txt', 'wb+');
fclose($fp);

try {
  var_dump(file_descriptor($fp));
} catch ( ... ) {
  ...
}

[bukka]

In general PHP_STREAM_AS_FD should be supported if PHP_STREAM_AS_FD_FOR_SELECT which should be the case for all core stream wrappers - I just checked and don't see a case where it would be different. So I wouldn't bother with checking PHP_STREAM_AS_FD_FOR_SELECT as well.

[bukka]

It looks good in general. The test failure seems related as you probably know.