geo-distance.php

<?php
define( 'EARTH_RADIUS_MILES',      3959 );
define( 'EARTH_RADIUS_KILOMETERS', 6371 );
/**
 * SLC = Spherical Law of Cosines
 */ 
function distance_slc( $a, $b, $radius = EARTH_RADIUS_MILES ) {
	$delta_lat = $b['lat'] - $a['lat'];
	$delta_lng = $b['lng'] - $a['lng'];
	$distance  = sin( deg2rad( $a['lat'] ) ) * sin( deg2rad( $b['lat'] ) ) + cos( deg2rad( $a['lat'] ) ) * cos( deg2rad( $b['lat'] ) ) * cos( deg2rad( $delta_lng ) ) ;
	$distance  = acos( $distance );
	$distance  = rad2deg( $distance );
	$distance  = $distance * 60 * 1.1515;
	$distance  = round( $distance, 4 );
	return $distance;
}
function distance_haversine( $a, $b, $radius = EARTH_RADIUS_MILES ) {  
	$delta_lat = $b['lat'] - $a['lat'];
	$delta_lng = $b['lng'] - $a['lng'];
	$alpha     = $delta_lat/2;
	$beta      = $delta_lng/2;  
	$a1        = sin( deg2rad( $alpha ) ) * sin( deg2rad( $alpha ) ) + cos( deg2rad( $a['lat'] ) ) * cos( deg2rad( $b['lat'] ) ) * sin( deg2rad( $beta ) ) * sin( deg2rad( $beta ) ) ;
	$c         = asin( min( 1, sqrt( $a1 ) ) );
	$distance  = 2 * $radius * $c;
	$distance  = round( $distance, 4 );
	return $distance;
}
function distance( $a, $b, $radius = EARTH_RADIUS_MILES, $type = 'haversine' ) {
	switch( $type ) {
		case 'slc':
			return distance_slc( $a, $b, $radius );
			break;
		case 'haversine':
			return distance_haversine( $a, $b, $radius );
			break;
		default:
			# something to be added here
			return null;
			break;
	}
}