PHP程序中的错误，为什么不正确运行？

Question

<select name="filtering"> <option value="a">a</option> <option value="b">b</option>

if (isset($_POST['filtering']) == "a") {
    $stmt = prepare("SELECT * FROM user_posts ORDER BY post_time DESC LIMIT :limit OFFSET :offset");
}

if (isset($_POST['filtering']) == "b") {
    $stmt = prepare("SELECT * FROM user_posts ORDER BY post_time ASC LIMIT :limit OFFSET :offset");
}

Answer 1

Isset将仅返回true或false

if (isset($_POST['filtering'])&&($_POST['filtering']=="a")){
    $stmt = prepare("SELECT * FROM user_posts ORDER BY post_time DESC LIMIT :limit OFFSET :offset");
}

if (isset($_POST['filtering'])&&($_POST['filtering']=="b")) {
    $stmt = prepare("SELECT * FROM user_posts ORDER BY post_time ASC LIMIT :limit OFFSET :offset");
}