function find_angle ($x1, $y1, $x2, $y2) {
// This function takes two points (x1,y1) and (x2,y2)
// as inputs and finds the slope and angle of a line
// between those two points. It returns the angle
// and slope in an array. I can't figure out how to
// return a NULL value, so if the two input points
// are in a vertical line, the function returns
// $angle = 90 and $slope = 0. I know this is wrong.
if (($x2-$x1) != 0) {
$slope = ($y2-$y1)/($x2-$x1);
// Get rotation angle by finding the arctangent of the slope
$angle = rad2deg(atan($slope));
if ($x1 > $x2) {
$angle = 180+$angle;
} elseif ($y1 > $y2) {
$angle = 360+$angle;
}
} else {
// Vertical line has no slope, 90deg angle
$angle = 90;
# unset ($slope);
$slope = 0;
}
return array ($angle, $slope);
}