To search on non-textual fields you probably don't have to edit the config files. You just need to add the parameter correctly to your solr query.
The basic idea is you can use key/value pairs to search on non-textual fields. See http://wiki.apache.org/solr/SolrQuerySyntax.
To search on just s_number, you'd set up your query to the solr server like this:
This will return only results where s_number is 123, as expected -- and it does it as a regular search, not a post-search filter. I don't know what php solr library you're using, so I can't give you the exact code for how to set up this query parameter in your code. If you tell me what library you are using and/or post the source code that actually does the search, I might be able to give you more precise details.
But talking directly to your solr server, the query would look something like this: