php三下拉过滤器

我有一个名为classifieds的表，它包含以下内容：

• 身份证
• 头衔
• 描述
• 州id
• 城市标识
• 猫咪

这是从数据库动态填充的

<form name="theForm" method="post" action="advanced_search.php">
    <input type="submit" name="Submit" value="Search" />
    <select name="categories" >
        <option value=""> Select Category</option>
    </select>
    <select name="state" >
        <option value=""> Select state</option>
    </select>
    <select name="city" >
        <option value=""> Select city</option>
    </select>
    ...

---

有人能给我介绍一下吗？

只有在他们选择了以下内容时才添加where子句：

$select = "SELECT * FROM classifieds";
$where = " WHERE ( authorized = '1')";
if($categories) {
    $where .= " AND (cat = '" . $categories . "' )";
}
if($state) {
    $where .= " AND (state_id = '" . $state . "' )";
}
if($city) {
    $where .= " AND (city_id = '" . $city . "' )";
}
$q = $select . $where . ' ORDER BY date DESC';

---

要求“演练”并不适合堆栈溢出；你能把你的问题集中在当前阻碍你进步的具体问题上吗？你是否去掉了“

$where.=”和（cat=”、“$categories.”和state_id=”、“$state.”和city_id=”、“$city.”）？我编辑了我的答案，以提供生成查询的完整代码。仅当我选择类别、州和城市时，这才有效。但是如果我没有选择一个城市，那么结果显示为0。如果我只选择类别而不选择州和城市，则相同。您没有更改表单代码，是吗？我的代码假设“选择…”选项的值为空字符串。如果（$categories！=“”）{$where.=”和（cat=“$categories.”“；}等等，我就是这样做的。我整天都在玩这个，但我不能得到我想要的。
$select = "SELECT * FROM classifieds";
$where = " WHERE ( authorized = '1')";
if($categories) {
    $where .= " AND (cat = '" . $categories . "' )";
}
if($state) {
    $where .= " AND (state_id = '" . $state . "' )";
}
if($city) {
    $where .= " AND (city_id = '" . $city . "' )";
}
$q = $select . $where . ' ORDER BY date DESC';