PHP实现简单高精度的求PI方法

<?php
// pi = 2 + 2/3 + 2/3*2/5 + 2/3*2/5*2/7 + ...

$pi = (double)2.0; $z = (double)2.0;
$a = 1; $b = 3;

while ($z > 0.0000000000001) {
    $z *= $a / $b;
    $pi += $z;

    $b += 2;
    $a++;
}

echo $pi."\n";
echo "PHP PI() =>".pi()."\n";