Calc the radius (distance) between two points (2 latitudes and 2 longitudes)

calc_radius_between_two_points.php

<?php
  /**
  * @return integer 
  * @param double $lat_from latitude
  * @param double $lon_from longitude
  * @param double $lat_to latitude
  * @param double $lon_to longitude
  * @param string $unit The format of return ('km' =>kilometers,'mts'=>meters )
  *
  */
  public function getRadius($lat_from,$lon_from,$lat_to,$lon_to,$unit="km")
  {
    $R = 6371; // km (change this constant to get miles)
    $dLat = ($lat_to-$lat_from) * M_PI / 180;
    $dLon = ($lon_to-$lon_from) * M_PI / 180;
    $a = sin($dLat/2) * 
    sin($dLat/2) + cos($lat_from * M_PI / 180 ) * 
    cos($lat_to * M_PI / 180 ) * sin($dLon/2) * sin($dLon/2);
    $c = 2 * atan2(sqrt($a),sqrt(1-$a));
    $d = $R * $c;
    if($unit == 'mts'){
        return round($d*1000);//."m";
    }else{
        return round($d,3);//."km";
    }
  }
?>