In this tutorial, we will show you how to calculate the distance between two locations geolocated by using latitude and longitude in PHP. This distance calculation uses the **Spherical Law of Cosines**, which uses trigonometry to measure the curvature of the earth, to accurately measure the distances on the Earth.

```
function distance(lat1, lon1, lat2, lon2, unit) {
if ((lat1 == lat2) && (lon1 == lon2)) {
return 0;
}
else {
var radlat1 = Math.PI * lat1/180;
var radlat2 = Math.PI * lat2/180;
var theta = lon1-lon2;
var radtheta = Math.PI * theta/180;
var dist = Math.sin(radlat1) * Math.sin(radlat2) + Math.cos(radlat1) * Math.cos(radlat2) * Math.cos(radtheta);
if (dist > 1) {
dist = 1;
}
dist = Math.acos(dist);
dist = dist * 180/Math.PI;
dist = dist * 60 * 1.1515;
if (unit=="K") { dist = dist * 1.609344 }
if (unit=="N") { dist = dist * 0.8684 }
return dist;
}
}
```

The code above creates the function named **distance **to calculate the distance between two locations. It implies the simple spherical law of cosines that gives well-conditioned results down to distances as small as a few meters on the Earth’s surface. The distance function makes use of the spherical law of cosines formula `cos `

and derived into the distance calculation.*c* = cos *a* cos *b* + sin *a* sin *b* cos *C*

Parameters that are passed to the distance function are:**lat1**, **lon1** = Latitude and Longitude of point 1 in decimal degrees**lat2**, **lon2 **= Latitude and Longitude of point 2 in decimal degrees**unit **= the unit you desire for results where ‘M’ is the statute miles (default), ‘K’ is kilometers and ‘N’ is nautical miles