向mysql提交多维数组

我有一个带字段的mysql表:

• 预览URL
• 大URL

我有一个提交的对象,结构如下:

var $urls = {largeImg:[],preview:[]}

$urls.largeimg值必须插入到“大URL”中,并且

$urls.preview_url值必须插入“preview_url”

$urls.largeimg[0]必须与$urls放在同一个mysql表行中。preview[0],

$urls.largeimg[1]与$urls放在同一行中。预览[1]等。

我的php:

$urls = $_POST['urls'];

function cache_urls($urls){
global $db;


foreach($urls as $url){
    $sql = "INSERT INTO cache ";
    $sql .= "(preview_url, large_url) ";
    $sql .= "VALUES (";
    $sql .= "'" . db_escape($db, $url['preview']) . "', ";
    $sql .= "'" . db_escape($db, $url['largeImg']) . "'";
    $sql .= ");";

    $result = mysqli_query($db, $sql);
}

然后我也试了一下:

foreach($urls as $url){
    foreach($url as $key => $value){
        $sql = "INSERT INTO cache ";
        $sql .= "(preview_url, large_url) ";
        $sql .= "VALUES (";
        if($key==="preview"){
            $sql .= "'" . db_escape($db, $value) . "', ";
        }
        if($key==="largeImg"){
            $sql .= "'" . db_escape($db, $value) . "'";
        }

        $sql .= ");";

        $result = mysqli_query($db, $sql);
    }       
}

所以我认为sql位肯定是错误的,但我真的已经到了知识的尽头了!非常感谢您的帮助。