SortMode not working

Hi, im trying to sort my results by a UNIX_TIMESTAMP field, with no results, first i think that may be my sphinx conf or someting but i write a simple php script and works the sort by date

ive try t sort using this config

```
cl._sort = "fecha_ingreso";
cl._sort = { SPH_SORT_ATTR_DESC: 'fecha_ingreso' }
cl._sort = {'SPH_SORT_ATTR_DESC': 'fecha_ingreso' }
```

any ideas..

regards.
