http://cms.dev/article.php?post_id=6

http://cms.dev/article/6/

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^.]+)$ $1.php [NC,L]

RewriteEngine On

RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{DOCUMENT_ROOT}/$1.php -f [NC]
RewriteRule ^([^/]+)/([0-9]+)/?$ /$1.php?post_id=$2 [L,QSA]

RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{DOCUMENT_ROOT}/$1.php -f [NC]
RewriteRule ^(.+?)/?$ /$1.php [L]

RewriteEngine on
RewriteRule ^article/([0-9]+)$ article.php?post_id=$1
RewriteRule ^article/([0-9]+)/$ article.php?post_id=$1