使用JavaScript ajax在PHP中截断MySQL不起作用

I added a clear button to a site. This button should truncate the database. But after a click nothing happens.

Here is my code:

connection.php

<?php
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

Class dbObj{
    /* Database connection start */
    var $servername = "localhost";
    var $username = "root";
    var $password = "";
    var $dbname = "e";
    var $conn;
    function getConnstring() {
        $con = mysqli_connect($this->servername, $this->username, $this->password, $this->dbname) or die("Connection failed: " . mysqli_connect_error());

        /* check connection */
        if (mysqli_connect_errno()) {
            printf("Connect failed: %s
", mysqli_connect_error());
            exit();
        } else {
            $this->conn = $con;
        }
        return $this->conn;
    }
}

?>

response.php

<?php

ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

    //include connection file 
    include_once("connection.php");

    $db = new dbObj();
    $connString =  $db->getConnstring();

    $params = $_REQUEST;

    $action = isset($params['action']) != '' ? $params['action'] : '';
    $empCls = new Employee($connString);

    switch($action) {
     case 'clear':
        $empCls->clear();
     default:
     $empCls->getEmployees($params);
     return;
    }

    class Employee {
    protected $conn;
    protected $data = array();
    function __construct($connString) {
        $this->conn = $connString;
    }

    public function getEmployees($params) {

        $this->data = $this->getRecords($params);

        echo json_encode($this->data);
    }

    function clear() {

        $sql = "TRUNCATE `employee`";

        echo $result = mysqli_query($this->conn, $sql) or die("error to truncate employee data");
    }


    function getRecords($params) {
        $rp = isset($params['rowCount']) ? $params['rowCount'] : 10;

        if (isset($params['current'])) { $page  = $params['current']; } else { $page=1; };  
        $start_from = ($page-1) * $rp;

        $sql = $sqlRec = $sqlTot = $where = '';

        if( !empty($params['searchPhrase']) ) {   
            $where .=" WHERE ";
            $where .=" ( employee_name LIKE '".$params['searchPhrase']."%' ";    
            $where .=" OR employee_salary LIKE '".$params['searchPhrase']."%' ";

            $where .=" OR employee_age LIKE '".$params['searchPhrase']."%' )";
       }
       if( !empty($params['sort']) ) {  
            $where .=" ORDER By ".key($params['sort']) .' '.current($params['sort'])." ";
        }
       // getting total number records without any search
        $sql = "SELECT * FROM `employee` ";
        $sqlTot .= $sql;
        $sqlRec .= $sql;

        //concatenate search sql if value exist
        if(isset($where) && $where != '') {

            $sqlTot .= $where;
            $sqlRec .= $where;
        }
        if ($rp!=-1)
        $sqlRec .= " LIMIT ". $start_from .",".$rp;


        $qtot = mysqli_query($this->conn, $sqlTot) or die("error to fetch tot employees data");
        $queryRecords = mysqli_query($this->conn, $sqlRec) or die("error to fetch employees data");

        while( $row = mysqli_fetch_assoc($queryRecords) ) { 
            $data[] = $row;
        }

        $json_data = array(
            "current"            => intval($params['current']), 
            "rowCount"            => 10,            
            "total"    => intval($qtot->num_rows),
            "rows"            => $data   // total data array
            );

        return $json_data;
    }

}
?>

And finally the script from the script tag of the html:

<script type="text/javascript">
$( document ).ready(function() {
    var grid = $("#employee_grid").bootgrid({
        ajax: true,
        rowSelect: true,
        post: function ()
        {
            /* To accumulate custom parameter with the request object */
            return {
                id: "b0df282a-0d67-40e5-8558-c9e93b7befed"
            };
        },

        url: "response.php"
   });

function ajaxAction(action) {
                data = $(action).serializeArray();
                alert('alert 2');
                $.ajax({
                  type: "POST",  
                  url: "response.php",  
                  data: data,
                  dataType: "json",       
                  success: function(response)  
                  {
                    alert('alert 3');
                    $('#'+action+'_model').modal('hide');
                    $("#employee_grid").bootgrid('reload');
                  }                  
                });
            }
            $( "#command-add" ).click(function() {
                alert('alert 1');
              ajaxAction('clear');
            });
});
</script>

To see how far the functions get executed I've added the three alerts. "alert 3" never shows up.

I have tried many things:

-checking privileges of the MySQL user (I use XAMPP so user root has all permissions) -executing "truncate employee" in phpMyAdmin -> works -activating PHP error reporting: no error shows up

Maybe someone could solve this issue or has an idea.

Regards

Francis