无需操作即可提交html表单

I need to submit a form without reloading or even interrupting the page. The "submit" input should only start a php code where a sql INSERT query gets executed.

This is what I use at the Moment:

echo "
    <form method='post'>
        <input type='hidden' visibility='hidden' value='hiddenID' name='userid' id='userid'/>
        <textarea class='form-control' rows='5' name='newmessage' id='newmessage'></textarea><br/>
        <input class='btn btn-primary' type='submit' name='submit_chatmessage' value='Send'>
    </form>";


    if($_POST[newmessage] == ""){

    } else {

        date_default_timezone_set('Europe/Berlin'); // CDT

        $current_stamp = date('Y-m-d H:i:s');

        $sql="
        INSERT INTO tbl_messages (`tbl_users_ID`, `time`, `message`, `from_admin`) VALUES ($_POST[userid],'$current_stamp','$_POST[newmessage]',1)";

        mysql_query($sql);
        }
    }

I already tried things like an onsubmit handler or action="javascript:void(0);" but that wasn't working. There was no new data in the database so the query wasn't executed.

If I submit the form normally, the query gets executed as expected, but the page reloads.

You coud say, that the php script starts when the INSERT was successful and if the button got already pressed:

if((mysql_query($sql)) && (isset($_POST['submit_chatmessage']))) {
 ...
}

(mysql_query() should return true or false)