Check to see if a username is already registered

<?php include("include/config/connect.php"); ?>
<script language="text/javascript">
function display() 
{
	document.getElementById("available").innerHTML = window.form.username.value;
}
</script>

<form onKeyUp="display()" name="form">
	<input type="text" name="username">&nbsp;

<?php 
$available = "<div id='available'></div>";

$sql = mysql_query("SELECT * FROM users WHERE username='$available'");
$result = mysql_num_rows($sql); 
			
if ($result == "1")
	{
		echo "<img src='images/x.gif'>";
	}
	else
	{
		echo "<img src='images/check.gif'>";
	}
?>

</form>