How can we determine if a point lies within a polygon using PHP?

PHP Implementation for Locating Points Within a Polygon

Question:

Given an array of latitude and longitude coordinates representing a polygon and a point with similar coordinates, how can we determine if the point lies within the polygon?

Solution:

<br>$vertices_x = array(37.628134, 37.629867, 37.62324, 37.622424);    // x-coordinates of polygon vertices<br>$vertices_y = array(-77.458334,-77.449021,-77.445416,-77.457819); // y-coordinates of polygon vertices<br>$points_polygon = count($vertices_x) - 1;  // number of polygon vertices<br>$longitude_x = $_GET["longitude"];  // x-coordinate of test point<br>$latitude_y = $_GET["latitude"];    // y-coordinate of test point</p>
<p>if (is_in_polygon($points_polygon, $vertices_x, $vertices_y, $longitude_x, $latitude_y)){<br>  echo "Point is within polygon!";<br>}<br>else echo "Point is not within polygon";</p>
<p>function is_in_polygon($points_polygon, $vertices_x, $vertices_y, $longitude_x, $latitude_y)<br>{<br>  $i = $j = $c = 0;<br>  for ($i = 0, $j = $points_polygon ; $i < $points_polygon; $j = $i  ) {</p><pre class="brush:php;toolbar:false">if ( (($vertices_y[$i]  >  $latitude_y != ($vertices_y[$j] > $latitude_y)) &amp;&amp;
 ($longitude_x < ($vertices_x[$j] - $vertices_x[$i]) * ($latitude_y - $vertices_y[$i]) / ($vertices_y[$j] - $vertices_y[$i]) + $vertices_x[$i]) ) )
   $c = !$c;

}
return $c;
}

In this PHP code, the is_in_polygon function iterates through each edge of the polygon, checking if the point is on the "side" of the polygon where the y-coordinate of one vertex is greater than the test point's y-coordinate and the other vertex's y-coordinate is less than the test point's y-coordinate. If both of these conditions are true, the function checks if the x-coordinate of the test point is less than the line segment between the two vertices. If so, it signifies a crossing of the polygon boundary and adds or subtracts 1 from a counter. Finally, if that counter is nonzero, it indicates that the test point is within the polygon.

Additional Resources:

For further support, consider using the polygon.php class, which provides an additional function, isInside, for determining if a point is within a polygon.

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