expm1(): equivalent to 'exp(num) - 1'

<?Php
echo expm1(5); // Output 147.41315910258 
echo "<br>";
echo expm1(1); // Output 1.718281828459 
echo "<br>";
echo expm1(0); // Output is 0
echo "<br>";
echo expm1(-1); // Output is -0.63212055882856
echo "<br>";
?>

float expm1(float X);