Those are references, and they are similar to “pointers” in C or C++.
More information is in the PHP manual.
In this case, since $db_hosts
is empty, the construct $db_hosts[$db_item['hostid']]
will create a new array with an empty item on the index of $db_item['hostid']
and return the item’s reference, making $db_host
act as an ‘alias’ for whatever $db_hosts[$db_item['hostid']]
is currently.