<script type="text/javascript">
	document.onkeyup = KeyCheck;       
	function KeyCheck(e){
...

<?php
$text = mysql_real_escape_string($_POST['text']);
$query = sprintf("INSERT INTO $table (`user_id`, `post_text`) VALUES ($d, $s);",$user_id,$text);
...

<?php

$text = htmlentities($_POST['text']);
...