PHP浮点数精度与货币计算

PHP浮点数精度与货币计算

浮点数精度问题是所有编程语言都有的。0.1加0.2不等于0.3，这个经典问题在PHP里也存在。今天说说PHP中数字处理的正确方式。

浮点数精度问题。

```php
echo "0.1 + 0.2 = " . (0.1 + 0.2) . "\n";
var_dump(0.1 + 0.2 == 0.3);

$sum = 0.1 + 0.2;
$epsilon = 0.0001;
var_dump(abs($sum - 0.3) < $epsilon);
var_dump(round($sum, 2) == 0.3);
?>

BCMath扩展提供高精度计算。

```php
if (!extension_loaded('bcmath')) die("BCMath未安装\n");

$result = bcadd('0.1', '0.2', 2);
echo "0.1 + 0.2 = $result\n";

$a = '123.456';
$b = '78.91';
echo "加法: " . bcadd($a, $b, 4) . "\n";
echo "减法: " . bcsub($a, $b, 4) . "\n";
echo "乘法: " . bcmul($a, $b, 4) . "\n";
echo "除法: " . bcdiv($a, $b, 4) . "\n";
?>

货币计算以分为单位。

```php
class Money
{
private string $amount;
private int $scale;

public function __construct(string $amount, int $scale = 2)
{
$this->amount = $amount;
$this->scale = $scale;
}

public static function fromDecimal(string $amount): self
{
$parts = explode('.', $amount, 2);
$integral = $parts[0];
$fraction = str_pad(substr($parts[1] ?? '00', 0, 2), 2, '0');
return new self($integral . $fraction);
}

public function toDecimal(): string
{
$len = strlen($this->amount);
if ($len <= 2) {
return '0.' . str_pad($this->amount, 2, '0', STR_PAD_LEFT);
}
return substr($this->amount, 0, $len - 2) . '.' . substr($this->amount, $len - 2);
}

public function add(Money $other): self
{
return new self(bcadd($this->amount, $other->amount, 0));
}

public function subtract(Money $other): self
{
return new self(bcsub($this->amount, $other->amount, 0));
}

public function multiply(float $factor): self
{
return new self(bcmul($this->amount, (string)$factor, 0));
}

public function compare(Money $other): int
{
return bccomp($this->amount, $other->amount, 0);
}

public function __toString(): string
{
return $this->toDecimal();
}
}

$price = Money::fromDecimal('99.99');
$quantity = 3;
echo "单价: {$price}\n";
echo "数量: {$quantity}\n";
?>

round、ceil、floor的区别。

```php
$value = 3.14159;
echo "round: " . round($value, 2) . "\n";
echo "ceil: " . ceil($value) . "\n";
echo "floor: " . floor($value) . "\n";

$num = 1234567.89;
echo "格式化: " . number_format($num, 2) . "\n";
echo "法国格式: " . number_format($num, 2, ',', ' ') . "\n";
?>

浮点数比较的注意事项。

```php
// 不要直接比较浮点数
// 用epsilon或round

function floatEquals(float $a, float $b, float $epsilon = 0.0001): bool
{
return abs($a - $b) < $epsilon;
}
?>

PHP的数字处理需要注意浮点数精度、类型转换、大整数范围等问题。货币计算用BCMath扩展或整数分单位。比较浮点数用epsilon或round。掌握了这些，数字处理就不会出问题了。