How to geocode using nominatim with no webserver integration ?

This is not really a Nominatim specific question; if you have a little experience with PHP you can come up with generic answers to your question. For example, if you have a working Nominatim installation with the database loaded and all, you could run this from Nominatim's website directory:

php -r '$_GET["q"]="karlsruhe"; $_GET["format"]="jsonv2"; include("search.php");'

to call the standard search script and feed it with the search term "karlsruhe" and request a JSON response. Of course that's just a quick hack, and you would probably take search.php and modify it slightly so that it takes its search string from a file or directly from stdin.

From your other questions it appears that you might hope for a nicely documented, no-programming-required solution that will give you instant satisfaction. I am afraid such a solution does not exist; you will have to at least spend a little time reading up on PHP basics if you want to succeed.