Distance between two coordinates (latitude/longitude) in PHP

gistfile1.php

function distance($lat1 = 0, $lng1 = 0, $lat2 = 0, $lng2 = 0, $miles = true)
{
  $pi80 = M_PI / 180;
  $lat1 *= $pi80;
  $lng1 *= $pi80;
  $lat2 *= $pi80;
  $lng2 *= $pi80;
  
  $r = 6372.797; // mean radius of Earth in km
  $dlat = $lat2 - $lat1;
  $dlng = $lng2 - $lng1;
  $a = sin($dlat / 2) * sin($dlat / 2) + cos($lat1) * cos($lat2) * sin($dlng / 2) * sin($dlng / 2);
  $c = 2 * atan2(sqrt($a), sqrt(1 - $a));
  $km = $r * $c;
  
  return ($miles ? ($km * 0.621371192) : $km);
}

# Source: http://snipplr.com/view/2531/calculate-the-distance-between-two-coordinates-latitude-longitude/
# Reference for others languages: http://www.codecodex.com/wiki/Calculate_distance_between_two_points_on_a_globe

please i am new to programming
please, where do i insert the two coordinates and how do i see the results. a working example will go a long way. thanks.