atanh
    (PHP 4 >= 4.1.0)
atanh -- Inverse hyperbolic tangent
Description
float 
atanh ( float arg)
     Returns the inverse hyperbolic tangent of      arg, i.e. the value whose      hyperbolic tangent is arg.     
Note: This function is not implemented on Windows platforms.
     See also atan(), asinh()      and acosh().