if submitted query is valid and executed, but it return nothing then php3_sybase_query may return TRUE (but TRUE isn't valid resultset).
Relevant part of code (functions/sybase.c->php3_sybase_query)
--------------------------------
num_fields = dbnumcols(sybase_ptr->link);
if (num_fields<=0) {
       RETURN_TRUE;
}
--------------------------------
So sybase_query end OK (via return on line 3 of listed code), but calls to other functions (as sybase_num_rows) complain that 1 isn't valid resultset.