使用php&；从Cat显示子类别；AJAX

我需要通过使用ajax和php显示选择cat的子cat

先选猫

 <select style="font-family:'Droid Arabic Kufi', serif"  id='dep' name="dep_id" class="form-control m-bot15">
                       <option selected disabled hidden value=''></option>
                       <?php
                       $e = mysql_query("select * from dep");
                       while ($row = mysql_fetch_array($e)) {
                       extract($row);
                       echo"<option value='$id'>$dep_name</option>";
                       }
                       ?>
                       </select> 

国际单项体育联合会（国际单项体育联合会）{ $q=$_POST['q']

if ($q == '1') {
    $id = $_POST['id'];
    $arr = array(array(array()));
    $e = mysql_query("select * from sub where dep='" . $id . "'") or die('sql error');
    $i = 0;
    while ($row = mysql_fetch_array($e)) {
        $arr[0][$i] = $row['id'];
        $arr[1][$i++] = $row['sub_name'];
    }
    $json = json_encode($arr);
    print_r($json);
}

它不起作用我需要知道为什么

include("config.php");

if ($q == '1') {
    $id = $_POST['id'];
    $arr = array(array(array()));
    $e = mysql_query("select * from sub where dep='" . $id . "'") or die('sql error');
    $i = 0;
    while ($row = mysql_fetch_array($e)) {
        $arr[0][$i] = $row['id'];
        $arr[1][$i++] = $row['sub_name'];
    }
    $json = json_encode($arr);
    print_r($json);
}