A CakePHP model function to find and sort by the distance from a given a latitude and longitude coordinate with the option to restrict to a given radius.

AppModel.php

<?php 

App::uses('Model', 'Model');
App::uses('String', 'Utility');

class AppModel extends Model {

/**
 * @author Reed Dadoune
 * distanceQuery
 * A genral case distance query builder
 * Pass a number of options to this function and recieve a query
 * you can pass to either the find or paginate functions to get
 * objects back by distance
 * 
 * Example:
 * 	$query = $this->Model->distanceQuery(array(
 * 		'latitude' => 34.2746405,
 *		'longitude' => -119.2290053
 * 	));
 * 	$query['conditions']['published'] = true;
 * 	$results = $this->Model->find('all', $query);
 *
 * @param array $opts Options
 *                    - latitude The latitude coordinate of the center point
 *                    - longitude The longitude coordinate of the center point
 *                    - alias The model name of the query this is for
 *                    	defaults to the current model alias
 *                    - radius The distance to at which to find objects at
 *                    	defaults to false in which case distance is calculated
 *                    	only for the sort order
 * @return array A query that can be modified and passed to find or paginate
 */
	public function distanceQuery($opts = array()) {
		$defaults = array(
			'latitude'  => 0,
			'longitude' => 0,
			'alias'     => $this->alias,
			'radius'    => false
		);
		$opts = Set::merge($defaults, $opts);

		$query = array(
			'fields' => array(
					'*',
					'( 3959 * acos( cos( radians('.$opts['latitude'].') ) * cos( radians( '.$opts['alias'].'.latitude ) ) * cos( radians( '.$opts['alias'].'.longitude ) - radians('.$opts['longitude'].') ) + sin( radians('.$opts['latitude'].') ) * sin( radians( '.$opts['alias'].'.latitude ) ) ) ) AS distance' //formula based on: https://developers.google.com/maps/articles/phpsqlsearch_v3#findnearsql
				)
			),
			'order' => array('distance' => 'ASC')
		);

		if ($opts['radius']) {
			$longitudeLower = $opts['longitude'] - $opts['radius']  / abs(cos(deg2rad($opts['latitude'])) * 69);
			$longitudeUpper = $opts['longitude'] + $opts['radius']  / abs(cos(deg2rad($opts['latitude'])) * 69);
			$latitudeLower  = $opts['latitude']  - ($opts['radius'] / 69);
			$latitudeUpper  = $opts['latitude']  + ($opts['radius'] / 69);
			$query['conditions'] = array(
				String::insert(':alias.latitude  BETWEEN ? AND ?', array('alias' => $opts['alias'])) => array($latitudeLower,  $latitudeUpper),
				String::insert(':alias.longitude BETWEEN ? AND ?', array('alias' => $opts['alias'])) => array($longitudeLower, $longitudeUpper)
			);
			$query['group'] = sprintf('%s.id HAVING distance < %f', $opts['alias'], $opts['radius']);
		}
		
		return $query;
	}
}

This is a great piece of code indeed!
You can also add a parameter to switch from miles to kilometres (multiplying by 6371 instead of 3959). See https://developers.google.com/maps/articles/phpsqlsearch_v3#finding-locations-with-mysql

How do you actually go about adding this to your cake setup? I've never added a model function.