You appear to be trying to use the REMOTE_ADDR from the browser to determine whether someone has voted or not. There are (at least) two problems with this.

First, a single (external) IP is often shared by many users. In companies or student dorms and the like, using NAT to allow multiple internally networked machines to access the internet via a single connection or corporate firewall etc. This means that only the first user to vote from within such an IP sharing group would be permitted to vote.

The second is that many home users still use dial-up connections and get allocated a different IP (via DHCP) each time they connect. I am one such user. In order for me to vote many times under your current scheme, I simply have to dis- and re-connect and click the vote button again. There is very little you can do to stop this using the IP alone.

Almost any scheme will require you to use a userid/password and/or a cookie to validate the voter. The former is non-trivial to implement though a search on merlyn's web-site will turn up several hits for schemes of varying complexity. The latter will fall down if your users are the paranoid types (like me) that habitually run with cookies disabled except on sites I have some modicum of trust in.