acosh():inverse hyperbolic cosine

acosh(float X);

X is the input

<?Php
echo acosh(6); // Output 2.4778887302885
echo "<br>";
echo acosh(2.5); // Output 1.5667992369724
echo "<br>";
echo acosh(0); // Output NAN
echo "<br>";
echo acosh(2); // Output 1.3169578969248
echo "<br>";
echo acosh(-2.5); // Output NAN
echo "<br>";
echo acosh(-4); // Output NAN
?>

acosh returns an EDOM error if called with an argument less than 1.0