Sure, just add the ' class="current"' to the current page as you build your tablist. I'll need to see the menu code you're using now in order to tell you where to add it.
A more alarming issue though is the use of a raw $_GET variable in the building of a file name that gets pulled from your server. Yikes! The '.php' you tack on the end might save you here, but otherwise somebody could just do something like this...
/?searchoption=../../../../../../etc/passwd
That would be bad... very very bad.
At the very least, you should build an array of possible choices from a database table, compare the $_GET variable to that list, and default to the home page if a match is not found. Never let the user just decide what file gets included. They could start snooping around for a 'reset.php' file or something like that.