Understanding the distance relationships in a Longitude and Latitude equation from an SQL query - sql

Question

In a PHP program that I did not develop I am able to enter, via a form, distance from a given US zipcode (radius in miles) from which to do a proximity search.
Let's take, for example, the city Gastonia, NC with a zipcode of 28054 and a radius distance of 10 miles.
The PHP code generates the SQL query dynamically. Before it gets to that point it does its calculations behind the scenes and gives these values:
:minlat (Float) 35.084832880851
:maxlat (Float) 35.374297119149
:minlon (Float) -81.305653802747
:maxlon (Float) -80.951286197253
It also gives this distance:
:distance (Float) 16093.47
I cannot see or manipulate the code that generates these values given the distance I entered into the form. However, I can override the values of each of these calculated variables.
I understand the :minlat and :minlon, it's the central point of my zipcode. What I don't understand is, what is :distance and what relationship does it have to :maxlat and :maxlon?
What type of measurement is :distance given that it started as 10 miles in the form?
Obviously :distance added to :maxlat or :maxlon doesn't make any sense.
What I ultimately want to be able to do is take a :minlat and :minlon, which I have a database of point, and then search a certain :distance.
So, if I wanted to search 20 miles, that would be :distance 32186 ish, but how does that affect maxlat and maxlon?
If you are interested in the entire SQL query, it's:
SELECT node.title AS node_title,
node.nid AS nid,
node.created AS node_created,
'node' AS field_data_field_item_photos_node_entity_type,
(COALESCE(ACOS(0.81684734668492*COS(RADIANS(location.latitude))*(0.15421945466762*COS(RADIANS(location.longitude)) + -0.9880366186544*SIN(RADIANS(location.longitude))) + 0.57685389156511*SIN(RADIANS(location.latitude))), 0.00000)*6370997.0816549) AS location_distance
FROM
{node} node
LEFT JOIN {location_instance} location_instance ON node.vid = location_instance.vid
LEFT JOIN {location} location ON location_instance.lid = location.lid
WHERE (( (node.status = '1')
AND (location.latitude > '35.084832880851'
AND location.latitude < '35.374297119149'
AND location.longitude > '-81.305653802747'
AND location.longitude < '-80.951286197253')
AND ((COALESCE(ACOS(0.81684734668492*COS(RADIANS(location.latitude))*(0.15421945466762*COS(RADIANS(location.longitude)) + -0.9880366186544*SIN(RADIANS(location.longitude))) + 0.57685389156511*SIN(RADIANS(location.latitude))), 0.00000)*6370997.0816549) < '16093.47') ))
Table structure
+-----------+---------------+------+-----+----------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+---------------+------+-----+----------+-------+
| zip | varchar(16) | NO | MUL | 0 | |
| city | varchar(30) | NO | | | |
| state | varchar(30) | NO | | | |
| latitude | decimal(10,6) | NO | MUL | 0.000000 | |
| longitude | decimal(10,6) | NO | MUL | 0.000000 | |
| timezone | tinyint(4) | NO | | 0 | |
| dst | tinyint(4) | NO | | 0 | |
| country | char(2) | NO | MUL | | |
+-----------+---------------+------+-----+----------+-------+