Force Non www to www in htaccess

 # Redirect non-www to www:

RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]

The contrary is also possible (www to non-www):

# Redirect www to non-www
RewriteCond %{HTTP_HOST} ^www\.(.*) [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L]