(PHP 4 >= 4.1.0, PHP 5, PHP 7, PHP 8)
expm1 —
返回 exp($num) - 1，甚至当 number 的值接近零也能计算出准确结果
$num): float
expm1() 返回“exp(num) - 1”，甚至当
num 的值接近零也能计算出准确结果。但是当两个数值趋近于相等的时候，“exp
(num) - 1”就会变得不太准确。
num要处理的参数
“e”的 num 次方减 1
Compare this to log1p (which is its inverse).
Also, You may have to use a similar workaraound in case the underlying C library
does not support expm1:
<?php
function expm1($x) {
return ($x>-1.0e-6 && $x<1.0e-6) ? ($x + $x*$x/2) : (exp($x)-1);
}
?>Note that exp(x)-1 can be approximated by x + x^2/2! + ... + x^n/n! and for any value