bcpowmod — Raise an arbitrary precision number to another, reduced by a specified modulus
Use the fast-exponentiation method to raise left_operand to the power right_operand with respect to the modulus modulus.
The left operand, as a string.
The right operand, as a string.
The modulus, as a string.
此可选参数用于设置结果中小数点后的小数位数。 也可通过使用 bcscale() 来设置全局默认的小数位数，用于所有函数。
Returns the result as a string, or NULL if modulus is 0.
Because this method uses the modulus operation, numbers which are not positive integers may give unexpected results.
The following two statements are functionally identical. The
bcpowmod() version however, executes in
less time and can accept larger parameters.
$a = bcpowmod($x, $y, $mod);
$b = bcmod(bcpow($x, $y), $mod);
// $a and $b are equal to each other.