htaccess RewriteEngine URL interpretation - php

Question

Lately I have changed the way my website works - physical page for every article vs. dynamically loaded content without physical (sub)page, but I realized I cannot simply upload the new site files cos I would break up all the social platform sharing links, counters and stuff as there are literally thousands of the subpages.
I heard (I know about it) that via .htaccess and RewriteEngine (I need using RewriteEngine as all the code in htaccess is made for it) I can make pages load internally something completely different depending on the actual URL, like, for example, if I have actual URL link to one of my subpages:
http://sub.mypage.com/php/somearticle.php?j=en
...so without changing the text of the URL it would load my new site files internally on different principle, like this:
http://sub.mypage.com/?s=somearticle&j=en
Now I also need that those variables "j" and the "somearticle" to be dynamic, or better said they need to be copied exactly as they are from the physical URL in the addressbar (where "somearticle" is actually name of the originally physical php file and the "j" is just language variable) as it will be something else every time so I do not have to make thousands of lines in htaccess for every single concrete subpage - I need some universal code that would manage all the subpages (as the principle is the same for all, just php names changes and sometimes language = variable "j"), you see?
So can anyone help me telling me the exact syntax/code for this to achieve?
EDIT
So I was playing with it myself a bit and this seems to work if I set the subpage manually (NOTE: just a clarification - this is for my localhost:8081 therefore I have in place that 1st condition cos for server I have different version that has different path to index.php) + I slightly updated variable j part thanx to #Ben's post:
# LOAD PAGE DIFFERENT INTERNALLY - LOCALHOST
RewriteCond %{HTTP_HOST} ^localhost:8081 [NC]
RewriteCond %{REQUEST_URI} /php/(.*)\.php$ [NC]
RewriteRule ^php/(.*).php$ /WWW/_PHP_/lego/index.php?s=$1 [QSA,L]
But unfortunately for some reason it affects every page on my site not only pages under /php/, so when I click to go to my first (default/initial = index.php) page it breaks it (it holds summary of all articles - they are not loaded) - anyone knows why, please?
SOLVED
So after small change to #Ben's code this is the right solution thus I take his solution as the right one (as it would actually work OK right away as it is if the page would be on server cos my test version is on my localhost where the path to /php/ directory is different))
LOAD PAGE DIFFERENT INTERNALLY - localhost
RewriteCond %{HTTP_HOST} ^localhost:8081 [NC]
RewriteCond %{REQUEST_URI} /php/([a-zA-Z0-9\-]+).php [NC]
RewriteRule ^ index.php?s=%1 [QSA,L]

Answer 1

REQUEST_URI is path component of the requested URI such as /php/somearticle.php, but not contains query string such as ?j=en.
The RewriteCond pattern, ! character (exclamation mark) is used to negate the result of the condition. To prefix with some pattern, use ^ character (caret), matches the beginning of a line.
%1 is the RewriteCond backreference that provides access to the grouped parts (in parentheses) of the pattern.
QSA flag, if the replacement URI contains a query string, the query string such as ?j=en will be appended to the newly rewrite uri.
RewriteCond %{REQUEST_URI} ^/php/([a-zA-Z0-9\-]+).php [NC]
RewriteRule ^ index.php?s=%1 [QSA,L]
See also: Apache Module mod_rewrite, RewriteRule Flags