First, the MySQL stuff:

<code lang="MySQL">
DELIMITER $

DROP FUNCTION IF EXISTS zip_deg2rad$
DROP FUNCTION IF EXISTS zip_gcd$
DROP FUNCTION IF EXISTS zip_distance$
DROP PROCEDURE IF EXISTS zip_radius$

CREATE FUNCTION zip_deg2rad(degrees DOUBLE) RETURNS DOUBLE
BEGIN
    RETURN degrees / (180 / PI()+0.000000000000000);
END$

CREATE FUNCTION zip_gcd(type ENUM('M', 'N', 'K'), src_lat DOUBLE, src_long DOUBLE, dst_lat DOUBLE, dst_long DOUBLE) RETURNS DOUBLE
BEGIN
    DECLARE temp DOUBLE;
    
    DECLARE STATUTE_MILES DECIMAL(5,1);
    DECLARE NAUTICAL_MILES DECIMAL(9,5);
    DECLARE KILOMETERS DECIMAL(5,1);
    
    SET STATUTE_MILES = 3963.0;
    SET NAUTICAL_MILES = 3437.74677;
    SET KILOMETERS = 6378.7;
    
    SET src_lat = zip_deg2rad(src_lat);
    SET src_long = zip_deg2rad(src_long);
    SET dst_lat = zip_deg2rad(dst_lat);
    SET dst_long = zip_deg2rad(dst_long);
    
    SET temp = ACOS(SIN(src_lat) * SIN(dst_lat) + COS(src_lat) * cos(dst_lat) * cos(dst_long - src_long));
    
    IF type = "M" THEN
        SET temp = STATUTE_MILES * temp;
    END IF;
    
    IF type = "N" THEN
        SET temp = NAUTICAL_MILES * temp;
    END IF;
    
    IF type = "K" THEN
        SET temp = KILOMETERS * temp;
    END IF;
    
    RETURN temp;
END$

CREATE FUNCTION zip_distance(type ENUM('M', 'N', 'K'), zip_start VARCHAR(5), zip_finish VARCHAR(5)) RETURNS DOUBLE
BEGIN
    DECLARE distance DOUBLE;
    
    DECLARE start_lat DOUBLE;
    DECLARE start_long DOUBLE;
    DECLARE finish_lat DOUBLE;
    DECLARE finish_long DOUBLE;
    
    SELECT latitude, longitude INTO start_lat, start_long FROM zipcodes WHERE zip = zip_start;
    SELECT latitude, longitude INTO finish_lat, finish_long FROM zipcodes WHERE zip = zip_finish;
    
    SELECT zip_gcd(type, start_lat, start_long, finish_lat, finish_long) INTO distance;
    
    RETURN distance;
END$

CREATE PROCEDURE zip_radius(IN type ENUM('M', 'N', 'K'), IN zip_start VARCHAR(5), IN radius INT, prec INT)
BEGIN
    DECLARE src_lat DOUBLE;
    DECLARE src_long DOUBLE;
    
    SELECT latitude, longitude INTO src_lat, src_long FROM zipcodes WHERE zip = zip_start;

    SELECT zip, city, state, county, ROUND(zip_gcd(type, src_lat, src_long, latitude, longitude), prec) AS `distance`
    FROM zipcodes
    WHERE zip != zip_start
    AND (POW((69.1 * (longitude - src_long) * COS(src_lat / 57.3)), 2) + POW((69.1 * (latitude - src_lat)), 2)) <= (radius * radius)
    ORDER BY `distance` ASC;
END$

DELIMITER ;
</code>

To use these, here are a few examples:

<li>As a note: for zip_gcd, zip_distance, and zip_radius; type is one of M (Miles), N (Nautical Miles), or K (Kilometers)</li>
<li>SELECT zip_gcd('M', 40.922326, -72.637078, 33.786594, -118.298662) AS `distance`;<br>Will return: 2529.2219424646 miles from Holtsville, NY to Palmdale, Ca</li>
<li>SELECT zip_distance('K', '28105', '00653');<br>Will return: 2346.5488627315 kilometers from Matthews, NC to Guanica, Peurto Rico</li>
<li>CALL zip_radius('N', '28105', 10, 0);<br>Will return 14 zip codes within a 10-nautical-mile radius from Matthews, NC (not including 28105), rounded to the nearest nautical mile</li>

Here is an example PHP script that will return all of the zip codes within a 10-mile radius from Charlotte, NC:

<code lang="PHP">
<?php
    $mysqli = mysqli_connect('localhost', 'dbuser', 'dbpass', 'zipcodes');

    if (mysqli_connect_errno()) {
        printf("Connection failed: %s\n", mysqli_connect_error());
        exit;
    }

    $sql = "CALL zip_radius('M', 28273, 5, 0)";

    if (!$mysqli->real_query($sql)) {
        printf("Invalid query: %s\nWhole query: %s\n", $mysqli->error, $sql);
        exit;
    }

    if ($result = $mysqli->store_result()) {
        while ($record = $result->fetch_assoc()) {
            $records[] = $record;
        }

        $result->close();
    }

    $mysqli->close();

    print_r($records);
?>
</code>

Just change the dbuser and dbpass to their respective credentials, and run this script to see the following output:

<code lang="Generic">
Array
(
    [0] => Array
        (
            [zip] => 28217
            [city] => CHARLOTTE
            [state] => NC
            [county] => MECKLENBURG
            [distance] => 2
        )

    [1] => Array
        (
            [zip] => 28134
            [city] => PINEVILLE
            [state] => NC
            [county] => MECKLENBURG
            [distance] => 4
        )

    [2] => Array
        (
            [zip] => 29716
            [city] => FORT MILL
            [state] => SC
            [county] => YORK
            [distance] => 5
        )

    [3] => Array
        (
            [zip] => 28210
            [city] => CHARLOTTE
            [state] => NC
            [county] => MECKLENBURG
            [distance] => 5
        )
)
</code>

It is very important to note that every single time you use zip_radius and zip_distance to remember to ALWAYS quote the zip codes. Since some of these zip codes start with a zero, not quoting them will produce NULL results from MySQL. This is because MySQL will treat them as numbers, and as such, numbers do not start with zeros. [smile]