More than one word tags

I’m working on a tags system. But when there is a more-than-one-word tag, on the query string instead of the space there is the space characters %20.

For example this tags on one article: php, mysql, php security, forms

displays like this:
php
mysql
php security
forms

the link for php security is something like www.domain.com/tag=php%20security

how do you handle this type of tags?