> http://php.net/manual/en/ref.bc.php
> http://php.net/manual/en/ref.gmp.php

编辑：Math_BigInteger

http://phpseclib.sourceforge.net/documentation/math.html的例子：

实现一个任意精度的整数运算库.否则,使用gmp或bcmath(如果有)和内部实现.

<?PHP
    include('Math/BigInteger.PHP');

    $a = new Math_BigInteger(2);
    $b = new Math_BigInteger(3);

    $c = $a->add($b);

    echo $c->toString(); // outputs 5
?>