if (isset($_REQUEST['query'])) {
    $query = $_REQUEST['query'];
    $sql = mysqli_query ($db, "SELECT a, b FROM table WHERE a LIKE '%{$query}%' OR b LIKE '%{$query}%'");
    $array = array();
    while ($row = mysqli_fetch_assoc($sql)) {
        $array[] = array (
            'label' => $row['a'].', '.$row['b'],
            'value' => $row['a'],
        );
    }
    //RETURN JSON ARRAY
    echo json_encode ($array);
}