title: Describe relative angles between points (like driving directions)
body: <p>I have a list of points with x, y coordinates. I know how to get the distance between points with <code>sqrt(pow($x2 - $x1, 2) + pow($y2 - $y1, 2))</code> and the angle between points with  <code>atan2(y1 - y2, x1 - x2)</code>.</p>
  
  <p>How can I calculate the relative angle between the points (left, right, straight)? So, if I'm at point 1, what is the relative direction to point 2, then 2 to 3, 3 to 4, etc...</p>
  
  <p>Thanks for any help!</p>