<?php

    $hostname = "localhost";
    $username = "***";
    $password = "***";
    $dbName = "***";


    mysql_connect($hostname, $username, $password) or die ("Не могу создать соединение");
    mysql_query('SET NAMES utf8') or header('Location: Error');

    mysql_select_db($dbName) or die (mysql_error());


     $MODEL = $_POST['model'];


    if (isset($MODEL) && !empty($MODEL)) {

    $query = "SELECT DISTINCT model FROM filtr_db";
    $DB = mysql_query($query);
    //  Обработаем
    $DB = array_column($DB, 'model');

   //  Выводим
    echo json_encode($DB);
   } else {
     echo '[]';
   }