//else if语句的应用
$moth = date("n"); //设置月份变量$moth
$today = date("j"); //设置日期变量$today
if ($today >= 1 and $today <= 10){ //判断日期变量是否在1-10之间
echo "今天是".$moth."月".$today."日上旬"; //如果是,说明是上旬
}elseif($today > 10 and $today <= 20){ //否则判断日期变量是否在11-20之间
echo "今天是".$moth."月".$today."日中旬"; //如果是,说明是中旬
}else{ //如果上面两个判断都不符合要求,则输出默认值
echo "今天是".$moth."月".$today."日下旬"; //说明是本月的下旬
}