Definition: We can use the Atan () function to figure out the arc tangent of a number. We can also use the Atan2 () function to figure out the arc tangent of a point given two coordinates. It is written as atan(Y,X) and calculates the arc tangent of Y/X. The result will always be between -PI and PI radians.
Also Known As: Arc Tangent, Arctangent
This will return:
<?php echo(atan2(5,5) . "<br />"); echo(atan2(-3,4)) ?>