There's actually a really good explanation of this where it was discussed recently in another thread, here:
Help with function stristr()

It's called the ternary operator, and aside from the explanation you'll read above, it's basically checking to see if $var is set, and if it has a value. If it does, it's reassigning the value back to itself ($var to $var), and if not, it's assigning a new value (the string 'new value') to $var.
-m