<?php

function cube_root($num)
{
    $a = $num;
    
    while(true)
    {
        $b = ($num/($a*$a) + $a*2) / 3;
        ++$i;
        $diff = abs($b - $a);
        if($diff < 1e-12)break;
        $a = $b;
    }
    return $b;
}

//test code
$ret = cube_root(10);
printf("%.16f\n%.16f\n", $ret, $ret*$ret*$ret);
$ret = cube_root(100);
printf("%.16f\n%.16f\n", $ret, $ret*$ret*$ret);