How to configure the size of the internal buffer for file IO in PHP?

Here is a workaround:

fsync($f);
$err = stream_set_write_buffer($f, 65536);

Why does it fail?

stream_set_write_buffer sets the buffer through setvbuf

if (data->file == NULL) {
    return -1;
}
...
return setvbuf(data->file, NULL, _IOFBF, size);

But PHP’s fopen function doesn’t call C’s fopen function, but instead calls the open system call, which returns a file descriptor, so this file field is a null:

static php_stream *_php_stream_fopen_from_fd_int(...)
{
    ...
    self->file = NULL;
    ...
}

Then I searched this file and found a function called php_stdiop_cast that sets the file field:

data->file = fdopen(data->fd, fixed_mode);

And this function is called by another function called php_stdiop_sync, which is the implementation of fsync and fdatasync functions.