Friendly URL and 301 redirect - .htaccess

Question

I'm trying to rewrite a URL so that it's user/search engine friendly, then 301 redirect the original URL to the new one.
At the moment the posts generate like this:
example.com/blog/post.php?s=nice-post
But I'd like them to look like this:
example.com/blog/nice-post
Here's what I've got so far:
RewriteEngine On
RewriteCond %{ENV:REDIRECT_STATUS} 200
RewriteRule ^ - [L]
RewriteCond %{QUERY_STRING} ^s=(.+)$
RewriteRule post.php /%1? [R=301,L]
RewriteCond %{REQUEST_URI} !post.php
RewriteRule ^([a-zA-Z0-9_-]+)$ post.php?s=$1
Unfortunately, this clashes with the 404 redirect, sending all pages not found to the blank post.php file, and I can't work out why. Is there a better way of approaching the URL rewrite/redirect?
Thanks for having a look :)

Answer 1

Elaborating on tdammers' suggestion about header()
in your post.php, before outputting anything, decide if it's a valid post or not (for example by looking its id up in the database).
if( !$valid ) {
header( 'HTTP/1.0 404 Not Found' );
echo "404";
exit;
}
i.e. if the page was found invalid, PHP sends out a 404 header to the browser to notify it that this is a missing page. Unfortunately, there's no straightforward way to redirect to Apache's default 404 page, so you'll have to output something yourself -- but since you already have a custom 404 page, you may just include() that.

Answer 2

Going to assume you've got these rules in an htaccess file in your /blog/ directory. Try adding a RewriteBase and removing the leading slash from your query string backreference:
RewriteEngine On
RewriteBase /blog/
RewriteCond %{ENV:REDIRECT_STATUS} 200
RewriteRule ^ - [L]
RewriteCond %{QUERY_STRING} ^s=(.+)$
RewriteRule post.php %1? [R=301,L]
RewriteCond %{REQUEST_URI} !post.php
RewriteRule ^([a-zA-Z0-9_-]+)$ post.php?s=$1