浮点数由于精度问题，不能直接进行比较。

$a = 100.411;$b = 100;$c = 0.411;$d = $a - $b;var_dump($c == $d);  // false

PHP提供了BC函数，BC Math

For arbitrary precision mathematics PHP offers the Binary Calculator which supports numbers of any size and precision, represented as strings.

其中bccomp用来比较两个任意精度的数值：

$a = 100.411;$b = 100;$c = 0.411;$d = $a - $b;var_dump(bccomp($c, $d, 3));  // 0 表示相等

Description

int bccomp ( string $left_operand = "" , string $right_operand = "" [, int $scale = int ] )

Compares the left_operand to the right_operand and returns the result as an integer.

Parameters

left_operand

The left operand, as a string.

right_operand

The right operand, as a string.

scale

The optional scale parameter is used to set the number of digits after the decimal place which will be used in the comparison. Default value 0.

Return Values

• 0 if the two operands are equal,
• 1 if the left_operand is larger than the right_operand,
• -1 otherwise.