Cygwin 3.3.6: PHP's SplFileObject::fwrite seems broken

[Hans Henrik Bergan]

running PHP 7.3.7 in Cygwin 3.3.6, and SplFileObject::fwrite seems
completely broken.
to reproduce:
$ touch bug.txt; chmod 0777 bug.txt; php -r '$o=new
SplFileObject("bug.txt");var_dump($o->fwrite("test"));var_dump(error_get_last());'
int(0)
NULL

so both fwrite() and error_get_last() failed to pick up that there was an
error,
but strace reveals:
67 487251 [main] php 1693 __set_errno: ssize_t write(int, const void*,
size_t):1350 setting errno 9
60 487311 [main] php 1693 write: -1 = write(3, 0x6FFFFFE65BF8, 4), errno 9

errno 9 probably means: 9 EBADF Bad file descriptor.