好吧,似乎PHP和BC lib有一些限制,在网上搜索后我发现这个

interesting article/code:

所以你应该使用这个功能：

function NRoot($num, $n) {

if ($n<1) return 0; // we want positive exponents

if ($num<=0) return 0; // we want positive numbers

if ($num<2) return 1; // n-th root of 1 or 2 give 1

// g is our guess number

$g=2;

// while (g^n < num) g=g*2

while (bccomp(bcpow($g,$n),$num)==-1) {

$g=bcmul($g,"2");

}

// if (g^n==num) num is a power of 2, we're lucky, end of job

if (bccomp(bcpow($g,$n),$num)==0) {

return $g;

}

// if we're here num wasn't a power of 2 :(

$og=$g; // og means original guess and here is our upper bound

$g=bcdiv($g,"2"); // g is set to be our lower bound

$step=bcdiv(bcsub($og,$g),"2"); // step is the half of upper bound - lower bound

$g=bcadd($g,$step); // we start at lower bound + step , basically in the middle of our interval

// while step!=1

while (bccomp($step,"1")==1) {

$guess=bcpow($g,$n);

$step=bcdiv($step,"2");

$comp=bccomp($guess,$num); // compare our guess with real number

if ($comp==-1) { // if guess is lower we add the new step

$g=bcadd($g,$step);

} else if ($comp==1) { // if guess is higher we sub the new step

$g=bcsub($g,$step);

} else { // if guess is exactly the num we're done, we return the value

return $g;

}

}

// whatever happened, g is the closest guess we can make so return it

return $g;

}

echo NRoot("18446744073709551616","64");

?>

希望这有用……