admin – Get_option use inside switch select option


I try to make inside theme custom admin panel a switch select option. but i cannot get the value of switch case. following is my code:

$bannerchange = $_POST('bannerchange');

if( get_option('bannerchange') != get_option($bannerchange)) {
            $bannerchange = update_option( 'bannerchange', get_option($bannerchange));

}

<?php if(isset($_POST('wphw_submit')) && $chk):?>
  <div id="message" class="updated below-h2">
    <p>Content updated successfully</p>
  </div>


  <?php endif;?>
      <form method="post" action="">

<select name="wphw_submit" onchange="this.form.submit()">
     <option value="page1"<?php if ($bannerchange == "page1") { echo " selected"; } ?>>Custom Departure</option>
     <option value="page2"<?php if ($bannerchange  == "page2") { echo " selected"; } ?>>Fixed Departure</option>
</select>
<noscript><input type="submit" value="Submit"></noscript>
 </div>
<input type="submit" name="wphw_submit" value="Save changes" class="button-primary" />
<?php

switch ($bannerchange) {
    case 'page2': ?>
        
        <p>two</p>

        <?php
        break;
    case 'page1':
        ?>
        <p>one</p>
        <?php
        break;
    }
    ?>

$bannerchange get always undefined.