Syntax of atanh
float atanh(float value)
Returns the inverse hyperbolic tangent of value.
<?php
echo(atanh(M_PI_4) . "<br>");
echo(atanh(0.50) . "<br>");
echo(atanh(-0.50) . "<br>");
echo(atanh(1) . "<br>");
echo(atanh(-1));
?>
1.0593061708232
0.54930614433405
-0.54930614433405
INF
-INF