(PHP 4 >= 4.1.0, PHP 5, PHP 7)
expm1 — Returns exp(number) - 1, computed in a way that is accurate even when the value of number is close to zero
expm1 ( float $arg ) : float
expm1() returns the equivalent to 'exp(arg) - 1' computed in a way that is accurate even if the value of arg is near zero, a case where 'exp (arg) - 1' would be inaccurate due to subtraction of two numbers that are nearly equal.
arg
The argument to process
'e' to the power of arg minus one
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https://www.php.net/manual/en/function.expm1.php