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