How to change $scriptpath variable

[techxpert]

I want to change $scriptpath variable when users login.
say someone comes to my site using our IP
http://XXX.XXX.XXX.XXX/forum and login so when VB show him the message

Thank you for logging in, username.

Click here if your browser does not automatically redirect you.

I want to replace something like this
$scriptpath=str_replace("XXX.XXX.XXX.XXX","www.dom ain.com",$scriptpath);

Not sure where to add this code?
Can someone help please.

PS:We can not use .htaccess for redirections.