PHP表单操作不起作用，并且不更新查询字符串

I'm trying to make a filter using a MySQL database and PHP for a school project, but whenever I press my filter button, it doesn't execute the action so the querystring is not updated.

The reason it's referring to my homepage is because of this block code:

if (empty($_GET['page'])) {
    $_GET['page'] = 'home';
}

if (empty($routes[$_GET['page']])) {
    header('Location: index.php');
    exit();
}

This is when there is no $GET['page'] passed, and it will just refer to my homepage.

So the problem probably lies with my form action, which is clearly correct: action="index.php?page=agenda"

<form class="filter_form" method="get" action="index.php?page=agenda">
    <div class="location">
        <p class="filter_by">filter by:</p>
        <label for="location">location</label>
        <select name="location" id="location">
            <option value="all">-- Locations --</option>
            <?php foreach($locations as $location): ?>
                <option value="<?php echo $location['location']; ?>"
                    <?php
                        if(!empty($_GET['location'])){
                            if($_GET['location'] == $location['location']){
                                echo ' selected';
                            }
                        }
                    ?>
                  >
                    <?php echo $location['location']; ?>
                </option>
            <?php endforeach; ?>
        </select>
    </div>

    <div class="checker_wrapper">
        <div>
            <input type="checkbox" name="check1" id="check1">
            <label for="check1">skills only</label>
        </div>
        <div class="bottom_row">
            <input type="radio" name="group" id="radio1">
            <label for="radio1">day</label>
        </div>
    </div>

    <div class="radio_wrapper">
        <div>
            <input type="checkbox" name="check2" id="check2">
            <label for="check1">in group</label>
        </div>
        <div class="bottom_row">
            <input type="radio" name="group" id="radio2">
            <label for="radio2">evening</label>
        </div>
    </div>
    <input class="button filter_button" type="submit" value="filter" />
</form>

When I press the filter button, I'm in the homepage and the querystring is like this: index.php?location=all, so the $_GET[location] for my project works. It's just not adding the correct page in the string.

Your form is overriding the query string parameters in your action attribute.

Instead of putting the query string in the action, add a hidden field

<form class="filter_form" method="get" action="index.php">
   <!-- ... the rest of your form code -->
    <input class="button filter_button" type="submit" value="filter" />
    <input type="hidden" name="page" id="page" value="agenda" />
</form>

You should add a hidden input field into the form. Instead attach it into the action string,

<input type="hidden" name="page" value="agenda" />

Add a hidden field:

<input type="hidden" name="page" value="agenda" />

The get parameter in your action is overruled by the method "GET".

You should use a hidden input element in your form rather than adding your parameters in the URL:

<input id="page" name="page" type="hidden" value="agenda">