<?php
$dbHost = 'localhost';
$dbUsername = 'user';
$dbPassword = '';
$dbName= 'bd';
$db = new mysqli($dbHost, $dbUsername, $dbPassword, $dbName);
$db->set_charset("utf8");
if ($db->connect_error) {
die("Connection failed: " . $db->connect_error);}?>
<form action="javascript:void(null);" method="post" id="formx" onsubmit="call()">
<div id="type_subject"> Тип организации <br />
<?php $query = $db->query("SELECT * FROM t_gs_type  ORDER BY gs_type_txt ASC");
 $rowCount = $query->num_rows;?>
<select name="type" id="typesubj">
<option value="">Виберите из списка</option>
<?php if($rowCount > 0){
while($row = $query->fetch_assoc()){
echo '<option value="'.$row['gs_type_id'].'">'.$row['gs_type_txt'].'</option>';}
}else{echo '<option value="">Список недоступный</option>';}?>
</select>
</div>
<input value="Send" type="submit">
</form>
<div id="results">Вывод</div>

function call() {
$("#typesubj").change(function(){
    $.ajax({
      type: 'POST',
      url: 'handler.php',
      data: {typesubj:$(this).val()},
      success: function(data) {
        $('#results').html(data);
      },
      error:  function(xhr, str){
  alert('Возникла ошибка: ' + xhr.responseCode);
      }
    });)}
}

<?php echo "<pre>";
print_r($_POST);
echo "</pre>";