PHP Manual . Prev . Next . 3.0.5 - 3.0.18 only, PHP 4 >= 4.0.0) atan2 -- arc tangent of two variables . float atan2 ( float y, float x) . This function calculates the arc tangent of the two variables x and y . It is similar to calculating the arc tangent of y / x , except that the signs of both arguments are used to determine the quadrant of the result. The function returns the result in radians, which is between -PI and PI (inclusive). ...