UK postcode error

'm trying to find the miles from km

Everything I try will give me the wrong outcome:

<?php
function calc_postcode_seperation($pcodeA,$pcodeB)

{

// PCODE A

$result=mysql_query("SELECT * FROM postcodes WHERE Pcode='$_REQUEST[postcode]' LIMIT 1");

$row=mysql_fetch_array($result);

$gridn[0]=$row[Grid_N];

$gride[0]=$row[Grid_E];

// PCODE B
$result=mysql_query("SELECT * FROM postcodes LEFT JOIN cars_agents ON postcodes.Pcode = cars_agents.postcode WHERE Pcode='$pcodeB' LIMIT 1");

$row=mysql_fetch_array($result);

$gridn[1]=$row[Grid_N];

$gride[1]=$row[Grid_E];

// TAKE GRID REFS FROM EACH OTHER.

$distance_n=$gridn[0]-$gridn[1];

$distance_e=$gride[0]-$gride[1];

// CALCULATE THE DISTANCE BETWEEN THE TWO POINTS

$hypot=sqrt(($distance_n*$distance_n)+($distance_e*$distance_e));

$text.='Distance between '.$pcodeA.' and '.$pcodeB.' is: '.round($hypot/1000*0.6214/100,2).' Miles Approx';

return $text;

}

//echo calc_postcode_seperation('B7','B21');

?>