Website: Classifieds website (users may put ads, search ads etc)
I plan to use SOLR for searching and then return results as ID nr:s only, and then use those ID nr:s
First, the definitions: a Solr/Lucene document is roughly the equivalent of a database row. An index is roughly the same as a database table.
I recommend trying to store all the classified-related information in Solr. Querying Solr and then the database is inefficient and very likely unnecessary.
Querying in a specific region would be something like q=cars+region:washington
assuming you have a region
field in Solr.
The Solr wiki has tons of good information and a pretty good basic tutorial. Of course this can always be improved, so if you find anything that isn't clear please let the Solr team know about it.
I can't comment on the PHP client since I don't use PHP.