<?php

$p="";                        

//定义圆周率的两种取值

define("PI1",3.14);

define("PI2",3.142);

//定义值的精度

$height = "低";

//根据精度返回常量名，将常量变成了一个可变的常量

if($height == "中"){

    $p = "PI1";

}else if($height == "低"){

$p = "PI2";

}

$r=1;

$area=$p*$r*$r;             

echo $area;

?>

结果为什么会为0