关于php评论回复无限极嵌套如何实现？已写基本代码，期待高手解惑

$comments = array (
    array (
        'id' => '3',
        'parent' => '0'
    ),
    array (
        'id' => '9',
        'parent' => '0'
    ),
    array (
        'id' => '1',
        'parent' => '3'
    ),
    array (
        'id' => '2',
        'parent' => '3'
    ),
    array (
        'id' => '5',
        'parent' => '1'
    )
    ,
    array (
        'id' => '7',
        'parent' => '1'
    )
);

$sub = array ();
foreach ($comments as $a) {
    if ($a['parent'] == 0) {
        foreach ($comments as $b) {
            if ($b['parent'] == $a['id']) {
                foreach ($comments as $c) {
                    if ($c['parent'] == $b['id']) {
                        $b['children'][] = $c;
                    }
                }
                $a['children'][] = $b;
            }
        }
        $sub[] = $a;
    }
}
echo &#39;<pre class="brush:php;toolbar:false">&#39;;
print_r($sub);

// 无限级分类
    $list = array(
        array('id'=>1, 'fid'=>0, 'title' => '中国'), 
        array('id'=>2, 'fid'=>1, 'title' => '江苏'),
        array('id'=>3, 'fid'=>1, 'title' => '安徽'),
        array('id'=>4, 'fid'=>8, 'title' => '江阴'),
        array('id'=>5, 'fid'=>3, 'title' => '芜湖'),
        array('id'=>6, 'fid'=>3, 'title' => '合肥'),
        array('id'=>7, 'fid'=>3, 'title' => '蚌埠'),
    );
 
    $new = array();
    foreach($list as $v){
        $new[$v['fid']][] = $v;
    }
 
    $i = 0;
    $j = 0;
    $a = true;
    $p[$i] = 0;
    $q[$j] = $i;
    while($a){
        $next = false;
        $i = $q[$j];
        $var = $new[$i];
        if(!isset($p[$i])){
            $p[$i] = 0;
        }
        if($p[$i] == count($var)){
            echo &#39;</ul>&#39;;
        }else{
            for($k=$p[$i]; $k<count($var);$k++){
                if($k == 0)
                    echo &#39;<ul>&#39;;
                echo &#39;<li>&#39;.$var[$k][&#39;title&#39;];
                $p[$i]++;
                if(isset($new[$var[$k][&#39;id&#39;]])){
                    $i = $var[$k][&#39;id&#39;];
                    $j++;
                    $q[$j] = $i;
                    $next = true;
                    break;
                }
                echo &#39;</li>&#39;;
                if($k == count($var)-1){
                    echo &#39;</ul>&#39;;
                }
            }
        }
        if($next){
            continue;
        }
        $j--;
        if($j < 0){
            break;
        }
    }
     
    exit;