PHP atan function

atan

(PHP 3, PHP 4, PHP 5)

atan — Arc tangentDescriptionfloat 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().