SetFilter
takes an array. In your example you are putting $cat_str
into an array. A array of one item.
So you just needs to build array with all the ids.
$cl->SetFilter( 'catid', array( $cat1, $cat2, $cat3 ));
But thats not very flexible. So you probably build the array dynamically, rather than hard-coded like that. But thats upto your application how to build the array.
But also storing the ids, in three sperate attributes, makes it hard to search. Notice in the above example, just noticed a attribute called catid
. This would be a single multi-value attribute, that contains the ids from all three cat fields. That way its easy to search for ids in ANY of the columns at once.
http://sphinxsearch.com/docs/current.html#mva
if using a sql source, could do with something like
sql_query = SELECT id, title ... , CONCAT_WS(',', CatID1, CatID2 and CatID3) as catid FROM ...
sql_attr_multi = uint catid from field;