Exponential
The exponential is a mathematical function. It satisfies the expressions: exp($a + $b) = exp($a) * exp($b), and exp(0) === 1.
There is a special expm1() function, which calculates the exponential value of $x - 1. When $x is close to 0, this function provides more significant digits than using the expression exp($x) - 1.
Exponential calculations may also be led with the constant M_E and the operator ** or the function pow(), although there is a slight difference.
<?php
$e = exp(1); // also known as the contant M_E
var_dump(exp(2) === M_E ** 2); // false
var_dump(exp(2) - M_E ** 2); // float(8.881784197001252E-16)
?>