找規律寫程序

有一組 1,1,2,3,5,8,13 ? 求第8位的數

function getNum($n){
	if($n<=0){
		return 0;
	}else if($n<=2){
		return 1;
	}else if($n>2){
		return getNum($n-1)+getNum($n-2);
	}
}
$num = getNum(8);
echo $num;