如何使用条件PHP在单击上运行SQL查询？

I am trying to create a room availability check page for a hostel and I am having an issue.

I have a database with a table named 'rooms' listing all type of rooms with these rows:

1. id [INT]
2. name (room type) [CHAR]
3. capacity (max capacity, not to be changed)[INT]
4. used (number of beds used, I want to change this dynamically!) [INT]

I created a code to generate the rooms list from the DB with PHP and I want the "+" and "-" buttons to either add or remove one unit in the used column for a specific room. How can I do this?

Here is my code:

           <!-- SOME HTML/PHP THAT WORKS -->    
            <?php if ($roomlist->num_rows > 0) {
                // output data of each row
                while($room = $roomlist->fetch_assoc()) {
                $roomid = $room["id"]; ?>
              <div>
                <!-- SOME OTHER HTML/PHP THAT WORKS -->

                  // THE ISSUE IS BELOW, IT SHOWS THE CORRECT AMOUNT BUT $room["used"] DOES NOT UPDATE
                  <div>
                    Used: <?php echo $room["used"] . " / " . $room["capacity"] ?>
                  </div>
                  <div>
                    <form action="" method="POST">
                    <input type="submit" name="remove" value="-" />
                    <input type="submit" name="add" value="+" />

                    <?php
                    if(isset($_POST['remove'])){
                      $remove_query = mysqli_query("UPDATE rooms SET used = used - 1 WHERE id = $roomid") or die(mysqli_error());
                    } elseif (isset($_POST['add'])){
                      $add_query = mysqli_query("UPDATE rooms SET used = used + 1 WHERE id = $roomid") or die(mysqli_error());
                    }

                    ?>
                    </form>
                  </div>
                            </div>
                        </div>
              <?php }
                } else {
                    echo "0 results";
                } ?>

If you set the action of your form to whatever the name of your code is (e.g., "rooms.php"), then move

if(isset($_POST['remove'])){
    $remove_query = mysqli_query("UPDATE rooms SET used = used - 1 WHERE id     = $roomid") or die(mysqli_error());
} elseif (isset($_POST['add'])){
    $add_query = mysqli_query("UPDATE rooms SET used = used + 1 WHERE id = $roomid") or die(mysqli_error());
}

up to the top so it updates the table before you query the table to fill in the rest of the page, it should work. Right now, your php is updating the table after the SELECT query to populate your page, so it doesn't appear to be updating.

I think a better tack would be implementing AJAX so your form updates the Used field without reloading the page.

Alright so I figured out a way on my own in the end so I post it here for other people facing a similar issue to overcome it ;)

Here is the concerned part of the code in index.php:

<form action="update.php?id=<?php echo $roomid ?>&action=remove&used=<?php echo $room["used"] ?>&capacity=<?php echo $room["capacity"] ?>" method="post">
<input type="submit" name="remove" class="minus" value="-" />
</form>
<form action="update.php?id=<?php echo $roomid ?>&action=add&used=<?php echo $room["used"] ?>&capacity=<?php echo $room["capacity"] ?>" method="post">
<input type="submit" name="add" class="plus" value="+" />
</form>

And the code of update.php:

if (isset($_REQUEST['used']) && isset($_REQUEST['id']) && isset($_REQUEST['capacity'])) {

    $roomid  = $_REQUEST['id'];
    $used = $_REQUEST['used'];
    $capacity = $_REQUEST['capacity'];

    if (isset($_REQUEST['action']) && $_REQUEST['action'] == 'add') {
      if ($used >= $capacity) {
        header("location: index.php");
      } else {
        $newValue = $used + 1;
        $add_query = mysqli_query($connection, "UPDATE `rooms` SET `used` = $newValue WHERE `ID` = $roomid") or die(mysqli_error());
        header("location: index.php");
      }
    } elseif (isset($_REQUEST['action']) && $_REQUEST['action'] == 'remove') {
      if ($used <= 0) {
        header("location: index.php");
      } else {
        $newValue = $used - 1;
        $remove_query = mysqli_query($connection, "UPDATE `rooms` SET `used` = $newValue WHERE `ID` = $roomid") or die(mysqli_error());
        header("location: index.php");
      }
    }
}