Using the jQuery-UI Autocomplete Widget

 
//we will be using this to cache the responses from the server
var ajaxCache = {};
 
//activate autocomplete on boxes that have the autocomplete class
$("input.auto_complete").autocomplete({
	source: function(request, response) {
		//what are we searching for
		var query_type = $(this).attr('element').attr('id');
		//the cacheterm that we use to save it in the cache
		var cachedTerm = (request.term + '' + query_type) . toLowerCase();
		//if the data is in the cache and the data is not too long, use it
		if (ajaxCache[cachedTerm] != undefined && ajaxCache[cachedTerm].length < 13) {
			//map the data into a response that will be understood by the autocomplete widget
			response($.map(ajaxCache[cachedTerm], function(item) {
				return {
					label: item.value,
					value: item.value
				}
			}));
		}
		//get the data from the server
		else {
			$.ajax({
				url: "/ajax/auto_complete.php",
				dataType: "json",
				data: {
					query_type: query_type,
					q: request.term
				},
				success: function(data) {
					//cache the data for later
					ajaxCache[cachedTerm] = data;
					//map the data into a response that will be understood by the autocomplete widget
					response($.map(data, function(item) {
						return {
							label: item.value,
							value: item.value
						}
					}));
				}
			});
		}
	},
	//start looking at 3 characters because mysql's limit is 4
	minLength: 3,
	//when you have selected something
	select: function(event, ui) {
		//close the drop down
		this.close
	},
	//show the drop down
	open: function() {
		$(this).removeClass("ui-corner-all").addClass("ui-corner-top");
	},
	//close the drop down
	close: function() {
		$(this).removeClass("ui-corner-top").addClass("ui-corner-all");
	}
});

 
<?php
<?php
 
//make sure that we only allow valid query types
/**
 * @var array
 */
$validQueryTypes = array(
	'country',
	'city',
	'first_name',
	'last_name',
);
/**
 * @var string
 */
$column = (isset($_POST['query_type']) && in_array($_POST['query_type'], $validQueryTypes))? $_POST['query_type'] : null;
/**
 * @var string
 */
$q = isset($_POST['q'])? $_POST['q'].'%':null;
 
 
if($column && $q){
	switch($column){
		case 'country':
		case 'city':
			$q = new sQuery();
			$results = $q->from('addresses')
				//to make things simpler for the javascript, always select as value
				->column($column . ' as value')
				->where($column, $q, 'LIKE')
				->getAll();
 
			//SELECT $column as value FROM addresses WHERE $column LIKE '$q%';
			break;
		case 'first_name':
		case 'last_name':
			$q = new sQuery();
			$results = $q->from('users')
				->column($column . ' as value')
				->where($column, $q, 'LIKE')
				->getAll();
			//SELECT $column as value FROM users WHERE $column LIKE '$q%';
			break;
	}
 
	//$result is something like this:
	array(
		array('value'=>'Canada'),
		array('value'=>'America'),
		array('value'=>'Mexico'),
		array('value'=>'Netherlands'),
	);
 
	//then return it to the javascript
	echo json_encode($results);
}
exit;

 
City:
<input type="text" name="city" id="city" title="enter a city" class="auto_complete" />
 
Country:
<input type="text" name="country" id="country" title="enter a country" class="auto_complete" />
 
First Name:
<input type="text" name="first_name" id="first_name" title="enter a first_name" class="auto_complete" />
 
Last Name:
<input type="text" name="last_name" id="last_name" title="enter a last_name" class="auto_complete" />