htaccess - Rewrite PHP $_GET only in root
RewriteEngine on

# ignore existing files (only apply to non-file)
RewriteCond %{REQUEST_FILENAME} !-f

#ignore folders (only apply to non-folder)
RewriteCond %{REQUEST_FILENAME} !-d

# here you might also want to exclude some specific locations...
RewriteCond %{REQUEST_URI} !^(images|css|js)/

# finally, apply the rule...
RewriteRule ^(w+)$ index.php?p=$1 [L]