最终效果图：

代码详情：

include('db.inc.php');
function likecate($path=''){
    sql = "select id,catename,path,concat(path,',',id) as fullpath from likecate order by fullpath asc";
    $res = mysql_query($sql);
    $result = array();
    while($row = mysql_fetch_assoc($res)){
        $deep = count(explode(',',trim($row['fullpath'],','))); //取数组的深度,去除逗号
        $row['catename'] = str_repeat('  ',$deep).'|--'.$row['catename'];
        $result[] = $row;
    }
    return $result;
}
$res = likecate();
echo "<select name= cate>";
foreach ($res as $key => $value) {
    echo "<option> {$value['catename']}</option>";
}
echo "</select>";
print_r($res);