OK, after looking at that circuit and reading the guy's text (don't people believe in paragraphs any more?), I'm convinced that circuit is nuts, beyond the errors that were pointed out.
Using LDRs will be problematic. This is because it's assuming that a person entering will cause enough light drop to run the circuit. not always true - sensor placement is critical and it will take some tweaking to handle all ranges of light. In addition, people sometimes bunch up when they enter or leave a room so the circuit could easily get confused. what if some one enters halfway, swings their arm when turning around and leaves? Again placement is critical. Also, this circuit needs light to function. Where's that light coming from if the purpose is to turn on the lights??? In general, this is simply the wrong way to do it. As was pointed out, a PIR is a better way, though not fool proof either.