Full disclosure, I did not write the following distance() PHP function. It’s based on this answer on Stack Overflow.

The last few sites I’ve worked on have needed to return the closest location to the users IP based on an array of latitudes and longitudes.

In order to do this we need to do two things. Find the latitude and longitude of a given IP address, and then do a comparison against other latitudes and longitudes.

To find the latitude and longitude of an IP address we’ll use the free geolocation lookup API, FreeGeoIP.

Start by creating a function, geo_ip():

function geo_ip() {
    
    $ip  = !empty($_SERVER['HTTP_X_FORWARDED_FOR']) ? $_SERVER['HTTP_X_FORWARDED_FOR'] : $_SERVER['REMOTE_ADDR'];

    $url = "http://freegeoip.net/json/{$ip}";
    $ch  = curl_init();
    
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 5);
    $data = curl_exec($ch);
    curl_close($ch);
    
    if ($data) {
        return json_decode($data);
    } else {
        return false;
    }

}

In the geo_ip() function, we start by creating a variable for the current IP address using a ternary operator.

We then set the $url variable to query the API using the IP we just set. You can have it return a CSV or XML instead of JSON if you prefer.

After that, cURL the API and if it returns data, return that from the function. Otherwise return false.

Next create the function closeset_location():

function closest_location($locations) {

    if($locations && is_array($locations)) {

        $ip_data = geo_ip();
        
        if ($ip_data) {
            
            $coords[] = $ip_data->latitude;
            $coords[] = $ip_data->longitude;
        
            function distance($a, $b) {
                list($lat1, $lon1) = $a;
                list($lat2, $lon2) = $b;
            
                $theta = $lon1 - $lon2;
                $dist = sin(deg2rad($lat1)) * sin(deg2rad($lat2)) +  cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * cos(deg2rad($theta));
                $dist = acos($dist);
                $dist = rad2deg($dist);
                $miles = $dist * 60 * 1.1515;
                return $miles;
            }
            
            $distances = array_map(function($item) use ($coords) {
                $a[] = $item['latitude'];
                $a[] = $item['longitude'];
                return distance($a, $coords);
            }, $locations);
            
            asort($distances);
            
            return $locations[key($distances)];
        }
    }

}

closest_location() requires an array that contains elements for both latitude and longitude. We then set a variable using the geo_ip() function we created earlier. If it returns a value, we set up a $coords array for the latitude and longitude returned from FreeGeoIP.

The distance() function requires 2 sets of latitudes and longitudes. It then compares the distance between the two coordinates on a sphere.

$distances uses the distance() function we just created to compare the latitudes and longitudes from the $locations array passed into the function. Then sort the array to get the closest location and return it!

Now, say you have a variable called $stores that is an array of latitudes and longitudes if you call closest_location($stores) it will return the closest location in the array!