From postcodes you need longitude and latitude. From this you can calc distance between two postcodes.
PHP Code:
<?php
function getDistance($lat1, $long1, $lat2, $long2)
{
//$earth = 6371; //km change accordingly
$earth = 3960; //miles
//Point 1 cords
$lat1 = deg2rad($lat1);
$long1= deg2rad($long1);
//Point 2 cords
$lat2 = deg2rad($lat2);
$long2= deg2rad($long2);
//Haversine Formula
$dlong=$long2-$long1;
$dlat=$lat2-$lat1;
$sinlat=sin($dlat/2);
$sinlong=sin($dlong/2);
$a=($sinlat*$sinlat)+cos($lat1)*cos($lat2)*($sinlong*$sinlong);
$c=2*asin(min(1,sqrt($a)));
$d=round($earth*$c,2);
return $d;
}
?>