Distance Between Two Latitude + LongitudePoints - PHP

I once had a project where the client wanted customers to be able to search for the nearest vendor to the customers zip code - this project was in PHP but the algorithim contained here can be translated to other languages pretty easily (Note, this returns the value in miles):

function computeDistance($lat1,$lon1,$lat2,$lon2){

 $lat1 =  deg2rad($lat1);
 $lon1 =  deg2rad($lon1);
 $lat2 =  deg2rad($lat2);
 $lon2 =  deg2rad($lon2);


// Find the deltas
      $delta_lat = $lat2 - $lat1;
      $delta_lon = $lon2 - $lon1;

// radius of earth in miles
 $r = 3963.1; 

      // Find the Great Circle distance
      $distance = pow(sin($delta_lat/2.0),2) + cos($lat1) * cos($lat2) * pow(sin($delta_lon/2.0),2);
      $distance = $r * 2 * atan2(sqrt($distance),sqrt(1-$distance)); 

 return $distance;



}