如何重写干净的URL并删除单词之间的空格

Question

my currrent url is :

http://codagu.com/Environmental impact

i'm trying to write like :

http://codagu.com/Environmental_impact

my link look like:

<li><a href="http://codagu.com/" ><span>'.str_replace("-"," ",$row['title']).'</span></a>

my htaccess code is like what should i change in that to make my url work and replace space with underscore

RewriteEngine On
RewriteRule ^\.html$ / [L]