PHP错误计数

Question

有人可以解释一下，为什么在我的代码65259.33026979 +（ - 65259.33026979）中返回-7.2759576141834E-12而不是零？

$nx3 = 616;
$nx2 = 65349;
$dzN1 = 6746;
$dzN = 982;

$Qn = $nx3 / $dzN1;
$w2n2 = ($Qn * $dzN)*(-1);

$w3n2 = $nx2 + $w2n2;
$Qn2 = $w3n2 / $dzN1;

$w4n2 = ($Qn2 * $dzN1)*(-1);

$w5n2 = $w3n2 + $w4n2;

echo '$w3n2 = '.$w3n2.'<br />';
echo '$w4n2 = '.$w4n2.'<br />';
echo $w3n2.' + ('.$w4n2.') = '.$w5n2;

当我正常尝试添加这两个值时，我得到了正确答案。