<?php

/**
 * Query builder scope to list neighboring locations 
 * within a given distance from a given location
 *
 * @param  Illuminate\Database\Query\Builder  $query  Query builder instance
 * @param  mixed                              $lat    Lattitude of given location
 * @param  mixed                              $lng    Longitude of given location
 * @param  integer                            $radius Optional distance
 * @param  string                             $unit   Optional unit
 *
 * @return Illuminate\Database\Query\Builder          Modified query builder
 */
public function scopeDistance($query, $lat, $lng, $radius = 100, $unit = "km")
{

    $unit = ($unit === "km") ? 6378.10 : 3963.17;
    $lat = (float) $lat;
    $lng = (float) $lng;
    $radius = (double) $radius;
    return $query->having('distance','<=',$radius)
                ->select(DB::raw("*,
                            ($unit * ACOS(COS(RADIANS($lat))
                                * COS(RADIANS(latitude))
                                * COS(RADIANS($lng) - RADIANS(longitude))
                                + SIN(RADIANS($lat))
                                * SIN(RADIANS(latitude)))) AS distance")
                )->orderBy('distance','asc');
}