Description:
------------
From File::_getFilePointer():

// Writeable?
} elseif ( (FILE_MODE_WRITE == $mode OR FILE_MODE_APPEND == $mode)
    AND !file_exists($filename)
    AND !is_writeable(dirname($filename))) {

    return PEAR::raiseError('Could not create file: ' . $filename);

} elseif ( (FILE_MODE_WRITE == $mode OR FILE_MODE_APPEND == $mode)
    AND !is_writeable(dirname($filename))) {

    return PEAR::raiseError('File is not writeable: ' . $filename);
}

I believe that the second elseif should read:

    AND !is_writeable($filename)) {

The file can have writable permissions even though the directory it's in does not.