(PHP 4 >= 4.0.2, PHP 5)
Sets certain flags of the given field.
fdf_document
The FDF document handle, returned by fdf_create(), fdf_open() or fdf_open_string().
fieldname
Name of the FDF field, as a string.
whichFlags
newFlags
如果成功则返回 TRUE,失败则返回 FALSE。