Distinct for array of fields in mongo-php

[Setti Mahesh]

I am trying to find distinct values in array.

mongo query: db.runCommand ( { distinct: "inventory", key: "item.sku", query: { dept: "A"} } )

php: $db->command(array("distinct" => "inventory",

                        "key" => "item.sku",

                        "query" => array("dept" => "A")));

I am running above php script but it is returning empty value.