You can use:
Options +FollowSymLinks
RewriteEngine On
RewriteRule ^recruit(.*)$ recruit.php?id=/$1 [QSA,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{DOCUMENT_ROOT}/$1\.php -f [NC]
RewriteRule ^(.+?)/?$ /$1.php [L]
2nd rule will add .php
extension only for URIs that are not file or directories and that are already valid php files.