$.ajax({
type: 'post',
url: "ajax.php."
data: { id:'your id here' } //your id is your select box selected id.
}).done(function() {
});
no lado do php:
$id = $_REQUEST['id'];
$json = array();
$sql = mysql_query("select * from yourdb where id = $id");
while ($row = mysql_fetch_assoc($sql)) {
$json[] = $row;
}
echo json_encode($json);
ajax ref:
http://api.jquery.com/jQuery.ajax/ json ref:
http://en.wikipedia.org/wiki/JSON 
