Use a self-defined function in select clause MySQL - php

Question

I'm trying to get make a query which gives me a list of stores sorted by how far they are from the current location. I'm working in php and using MySQL for my database.
To calculate the distance between 2 stores, I use the longitudes and latitudes from the 2 stores and derive the distance from it. This is contained in a self-defined function distance($lat1, $lng1, $lat2, $lng2). The result of this function is the distance in km.
I want to use this function to create an extra column in my query result so I can sort all the stores from the one most behind my current location to the one most far from my current location. Both functions are declared in the same file, but I do not get any result. Is it possible to call a function in the SELECT clause by declaring it the way I did?
function getSortedStores($cur_lat, $cur_lng)
{
$query = "SELECT Store.ID, Store.Name, distance($cur_lat, $cur_lng, Address.Latitude, Address.Longitude) AS Distance FROM Store INNER JOIN Address ON Store.ID=Address.StoreID ORDER BY Distance";
$result = mysql_query($query);
return $result;
}
function distance($lat1, $lng1, $lat2, $lng2)
{
$toRadians = M_PI / 180;
$lat1 *= $toRadians;
$lng1 *= $toRadians;
$lat2 *= $toRadians;
$lng2 *= $toRadians;
$r = 6371; // 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;
$km = round($km, 1);
return $km;
}

Answer 1

you can't use php function in mysql. for more detail about mysql User-Defined Function
see this
http://dev.mysql.com/doc/refman/5.1/en/adding-functions.html

Answer 2

You can't mix PHP and mySQL in that way, you can either do the calculation in mySQL with their math functions, or select the raw data and do the calculations based on the result set in PHP. But you cannot call a PHP function inside a mySQL query.
Alternatively, and assuming that your stores are not mobile. You can create a simple table table to store the distances between all of your stores. It takes up a little extra storage, but can potentially save you a fair bit of CPU cycles in the end.
TABLE distances
store1_id INT PK
store2_id INT PK
distance FLOAT
SELECT distance
FROM distances
WHERE (store1_id = $store1 AND store2_id = $store2)
OR (store1_id = $store2 AND store1_id = $store2)
LIMIT 1