PHP Distance between Coordinates

Calculating the approximate shortest distance between two points on a sphere is simply the calculation of the arc length between them along a great circle (aka Riemannian circle, is a circle on the surface of a sphere that spits it into two equal halves).

The haversine formula is pretty useful in calculating this distance, and since the Earth is almost a sphere, it can provide a good approximation of distance between two coordinates. Enjoy!

function greatCircleDistance($latA, $lonA, $latB, $lonB, $radius = 6371)
{
 $latA = deg2rad($latA);
 $latB = deg2rad($latB);
 $lonA = deg2rad($lonA);
 $lonB = deg2rad($lonB);
 
 $deltaLat = $latB - $latA;
 $deltaLon = $lonB - $lonA;

 $sinDeltaLat = sin($deltaLat/2);
 $sinDeltaLon = sin($deltaLon/2);

 $rhs = $sinDeltaLat * $sinDeltaLat + cos($latA) * cos($latB) * $sinDeltaLong * $sinDeltaLong;

 $distance = $radius * 2 * asin(min(1,sqrt($rhs)));

 return $distance; // Returns distance in kilometers 
}

In case it was unclear,

$latA = Latitude of coordinate A
$lonA = Longitude of coordinate A
$latB = Latitude of coordinate B
$lonB = Longitude of coordinate B

$radius = Radius of sphere (in the above example, Earth's radius of 6371 km)

In whatever units you supply $radius will be the units in which the distance is returned by the function (i.e. if you supply miles, it will return distances in miles, meters in meters and so on....).

The formula is generally fairly accurate, but it does not take into consideration the road structure of the area (for example if the road has many bends you will travel a bit more), therefore it is likely to provide an underestimate of road travel distance. As expected, it is much more accurate for air travel, as planes tend to fly along great circles.