alternate:
$pi = 4 * atan(1); // 3.1415926535898
(PHP 4, PHP 5, PHP 7)
pi — 得到圆周率值
返回圆周率的近似值。返回值的 float
精度是由 php.ini 中的 precision
指令确定。默认值是 14。你也可以使用
M_PI 常量，该常量产生与 pi() 完全相同的结果。
圆周率（pi）的浮点近似值。
Example #1 pi() 例子
<?php
echo pi(); // 3.1415926535898
echo M_PI; // 3.1415926535898
?>
alternate:
$pi = 4 * atan(1); // 3.1415926535898
Pi is often usefull in con/sin/tan functions..
There are also other Pi related constants.
These are most of them:
M_PI = 3.14159265358979323846 // pi
// The following were added in PHP 4.0.0
M_PI_2 = 1.57079632679489661923 // pi/2
M_PI_4 = 0.78539816339744830962 // pi/4
M_1_PI = 0.31830988618379067154 // 1/pi
M_2_PI = 0.63661977236758134308 // 2/pi
M_SQRTPI = 1.77245385090551602729 // sqrt(pi) (Only in PHP 4.0.2+)
M_2_SQRTPI = 1.12837916709551257390 // 2/sqrt(pi)