无法访问amazon ec2上的mysql

I have 2 amazon ec2 instances.I want to connect from one to another. One has a Mysql database (instance 1). When I run the php code below on instance 2. I get a

ec2-x-x-x-x.us-west-2.compute.amazonaws.com is currently unable to handle this request.

But when I run the same code on http://phpfiddle.org/ the code works. Why does it not work on my server?

<?php

// On WEB_SERVER    
$host="public Ip of instance 1";              // Host name
$username="test";    // Mysql usernam
$password="pass";           // Mysql password
$db_name="mydab";             // Database name
$db_port="3306";   

// Create connection
$conn = new mysqli($host, $username, $password, $db_name,$db_port);
 Check connection
if ($conn->connect_error) {
    die('Connect Error (' . $mysqli->connect_errno . ') '
            . $mysqli->connect_error);
}else
{
    echo "connected";
}    
?>