how to include this external function? (php)

Hi, sorry about the silly question again, but where would I include this little bit of code so when that it can be used by my template and function properly.

Code:

 function toggle_width() {        var e = document.getElementById("gwrapper");
       if(e.clientWidth == '1000') {
          e.style.width = '89%';
          gsetCookie("sitewidth","89%",60);
       } else {
          e.style.width = '1000px';
          gsetCookie("sitewidth","1000px",60);
       }
    }

I think its a .php code / function ? So where exactly should I include it so that it can be used / called by my template code when ever, without it having to interfering or hitting my vb forums performance?

Thanks

that's not php... it's javascript.

wrap it in <script> tags and put it in the template before it's needed.