PHP错误计算10 - 9.99 - 为什么以及解决方案是什么？

Question

我在两台计算机（windows和linux）上重复了这个

    echo "<br />1. ". (10 - 9.99);
    echo "<br />2. ". (10 - 9.999);
    echo "<br />3. ". (10 - 9.9999);

    echo "<br />4. ". (10.0 - 9.99);
    echo "<br />5. ". (10.0 - 9.999);
    echo "<br />6. ". (10.0 - 9.9999);

    echo "<br />7. ". (10 - floatval(9.99));
    echo "<br />8. ". (10 - doubleval(9.99));

输出如下：

1. 0.0099999999999998
2. 0.00099999999999945
3. 9.9999999999767E-5
4. 0.0099999999999998
5. 0.00099999999999945
6. 9.9999999999767E-5
7. 0.0099999999999998
8. 0.0099999999999998

10 - 9.99 = 0.01

PHP 5.6.30-12~ubuntu16.04.1 + deb.sury.org + 1