switch $_GET variable else statement

Question

I've runned into a problem. If it's possible, can i write an else statement for the $_GET['topic'] ?

case 'thread-view':
        if(isset($_GET['topic'])){
        $topicid = $_GET['topic'];

        $psql = "SELECT * FROM posts WHERE th_p_link='$topicid' ORDER BY p_id ASC";
        $presult = mysqli_query($db,$psql);
        while($prow = mysqli_fetch_array($presult)){

            $post = $prow['p_post'];
            $author = $prow['p_author'];
            $avatar = $prow['p_avatar'];
            $gm = $prow['p_gm'];
            $date = $prow['p_date'];
            $link = $prow['th_p_link'];

        echo '
        <div class="post_box">
            <div id="post-left">
                <img src="'.$avatar.'" alt="">
                <div id="post-about">
                    <span>'.$author.'</span>
                    <span>Member</span>
                </div>
            </div>
            <div id="post-content">
            <p>'.htmlspecialchars($post, ENT_QUOTES).'</p>
            </div>
            <div id="post-right">
            <i>'.$date.'</i>
            </div>
        </div>
        ';
        }
        }
    break;

    default:
    include 'template/forum_categories.php';

For an example, if i go to the URL

forums.php?page=thread-view&topic=testurlpost

It shows the page correctly. Though what i'm trying to achieve is that if i go to something that doesn't exist such as

forums.php?page=thread-view&topic=testurlthatdoesntexist

I want it to re-direct to another page, such as an error page. How would i achieve this? I made an else statement for the $_GET['topic'] but that did not work when i entered the non-existing URL.

Answer 1

You should add

   if(!mysqli_num_rows($presult)) {
       header("Location: /error-page.html");
    }

before While statement. So your code will look something like this:

case 'thread-view':
    if(isset($_GET['topic'])){
    $topicid = $_GET['topic'];

    $psql = "SELECT * FROM posts WHERE th_p_link='$topicid' ORDER BY p_id ASC";
    $presult = mysqli_query($db,$psql);

    if(!mysqli_num_rows($presult)) {
       header("Location: /error-page.html");
    }

    while($prow = mysqli_fetch_array($presult)){