(PHP 4, PHP 5)
atan — Arc tangent
$arg
    )
   Returns the arc tangent of arg in radians.
   atan() is the complementary function of
   tan(), which means that
   a==tan(atan(a)) for every value of a that is within
   atan()'s range.
  
argThe argument to process
   The arc tangent of arg in radians.