Xmlhttp and php communication

I dont understand why when i post to a php script on the same page, which i just loaded it says 403. please help. here’s the page …

stuff
  <br><br>
    &nbsp &nbsp <button id="b1" onclick="do_get()">get</button>
    &nbsp &nbsp <button id="b2" onclick="do_post()">post</button>
<?php //var_dump ($_SERVER["REQUEST_METHOD"]); if ($_SERVER["REQUEST_METHOD"] == "POST") { var_dump($_POST); foreach($_POST as $key => $value) echo "
$key => $value"; } if ($_SERVER["REQUEST_METHOD"] == "GET") { var_dump ($_SERVER["QUERY_STRING"]); } ?>