PHP. Руководство по PHP. 2005
atan
atan
Description
float atan (float 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.
See also: atanh(), asin(), and acos().
atan2
atanh