PHP calculate lan/lon distance performance - php

Question

I have serious performance issues with a distance calculation script.
I have approximately 3000 locations (and this will eventually be doubled) in a database. The database structure is quite complex (categories, subcategories) but with time(); I saw that these query's didn't took much time.
I have a $_GET of latitude and longitude of the user and I use this calculation to determine if the location is within a certain radius:
function distance($lat1, $lon1, $lat2, $lon2, $unit) {
$theta = $lon1 - $lon2;
$dist = sin(deg2rad($lat1)) * sin(deg2rad($lat2)) + cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * cos(deg2rad($theta));
$dist = acos($dist);
$dist = rad2deg($dist);
$miles = $dist * 60 * 1.1515;
$unit = strtoupper($unit);
if ($unit == "K") {
return ($miles * 1.609344);
} else if ($unit == "N") {
return ($miles * 0.8684);
} else {
return $miles;
}
}
// some sql queries to get the lat/lon from the locations
if ((distance($_GET["lat"], $_GET["long"], $row3["content"], $row4["content"], "K") . "") < 10) {
//push to multidimensional array
}
$row3["content"] and $row4["content"] are the latitude and longitude values. For 3000 locations, this calculation takes up to 13 seconds!
I read this:
Fastest Way to Find Distance Between Two Lat/Long Points
I think the option to draw a box, based on the $_GET of latitude and longitude could perhaps already remove the current calculation. In the sql queries I can already filter out the locations outside the 10 km range.
But I have 2 questions:
If I change the SQL to something like this: ... WHERE LAT >= x1 AND <= x2, does this affect the time of the query?
In the explanation the writer talks about "units". I've been playing around with the lat/lon values, but how do I actually calculate x1, x2, y1, y2 where the $_GET value is a point in the center with a distance of 10 km?
Thank you.

Answer 1

I was able to reduce the calculation time from 13 seconds to 1 second!
I did this by filtering out with mysql the locations that were not within a 10 km bounding box of my lat/long coordinates.
I used this code:
$rad = 10; // radius of bounding circle in kilometers
$R = 6371; // earth's mean radius, km
// first-cut bounding box (in degrees)
$maxLat = $_GET['lat'] + rad2deg($rad/$R);
$minLat = $_GET['lat'] - rad2deg($rad/$R);
// compensate for degrees longitude getting smaller with increasing latitude
$maxLon = $_GET['long'] + rad2deg($rad/$R/cos(deg2rad($_GET['lat'])));
$minLon = $_GET['long'] - rad2deg($rad/$R/cos(deg2rad($_GET['lat'])));
and I changed my mysql to this:
$sql2 = "SELECT ....WHERE LAT BETWEEN '".$minLat."' AND '".$maxLat."'";
$sql3 = "SELECT ....WHERE LON BETWEEN '".$minLon."' AND '".$maxLon."'";
The rest of my code and calculation is exact the same, but instead of doing 3000 calculations, mysql sweeps out the majority.
I don't know if this approach is 100% mathematically correct, but as far as I see it works very fast with minor changes to my initial coding so for my project it's great.
And of course, the source: http://www.movable-type.co.uk/scripts/latlong-db.html