if(条件表达式1){

            PHP语句1;

}elseif(条件表达式2){

            PHP语句2;

}...........

else{

          PHP语句 ;

}

<?php

header("Content-type:text/html;charset=utf-8");    //设置编码

$mouth=date("n");             //设置月份变量$mouth

$today=date("j");             //设置日期变量$today

if($today>=1 and $today<=10){                         //判断日期变量是否在1~10之间

    echo  "今天是".$mouth."月".$today."上旬";         //如果是，说明是上旬 

}else if($today>=10 and $today<=20){                   //判断日期变量是否在11~20之间

    echo "今天是".$mouth."月".$today."中旬";            //如果是，说明是中旬

}else{                                                  //如果上面的两个判断都不符合要求，则输出默认值

    echo "今天是".$mouth."月".$today."下旬";             //上面本月是下旬

}

?>