Find someone using linkex, note down his url.
Make a simple webpage, containing just one link to his url.
Open his linkex, enter the url of the webpage you made, and for an anchor, try
Code:
<?php echo 'hi'; ?>
Check out his links, especially if he auto-adds stuff.
You can take it from there, I guess, I'm not going to publish working exploits. The linkex people need to fix their script, I would have provided a fix but not for such bullshit code without indentation.
Right now, running linkex = running a rootkit. Beware.