Unable to connect to AWS RDS through PDO

I set up a MySQL RDS instance in AWS. After instantiating the instance I tried to connect it from my EC2's command line and it works with no issue. However, when I try connecting through my PHP code I get erros. Below is the sample code that I tried to test my connectivity.

<?php
try{
    $dbh = new pdo( 'aws-test-db.hibizibi.us-west-2.rds.amazonaws.com;dbname=test',
                    'root',
                    'password',
                    array(PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION));
    die(json_encode(array('outcome' => true)));
}
catch(PDOException $ex){
    die(json_encode(array('outcome' => false, 'message' => 'Unable to connect')));
}

I get {"outcome":false,"message":"Unable to connect"}. My original code is using idiorm (which uses PDO). The portion that connects with database looks like below:

# configure ORM
ORM::configure('mysql:host='.DB_SERVER.';dbname='.DB_NAME);
ORM::configure('username', DB_SERVER_USERNAME);
ORM::configure('password', DB_SERVER_PASSWORD);

From the above I get the below error trace:

SQLSTATE[HY000] [2005] Unknown MySQL server host 'aws-test-db.hibizibi.us-west-2.rds.amazonaws.com:3306' (11)
#0 /var/www/html/main/admin/applications/vendors/idiorm/idiorm.php(255): PDO->__construct('mysql:host=aws-...', 'root', 'password', NULL)
#1 /var/www/html/main/admin/applications/vendors/idiorm/idiorm.php(237): ORM::_setup_db('default')

UPDATE

I updated my test code to try connecting using mysql extension and it worked with mysql but not PDO

<?php 

$servername = "aws-test-db.hizibizi.us-west-2.rds.amazonaws.com"; 
$username = "root"; 
$password = "password"; 
$dbname='test'; 


try{ 
    $dbh = new pdo(
        'mysql:'.$servername.';dbname='.$dbname.';',
        $username,
        $password,
        array(PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION)
    );

    die(json_encode(array('outcome' => true))); 
} catch(PDOException $ex) { 
    echo json_encode(array('outcome' => false, 'message' => $ex->getMessage()))."\n"; 
} 

// Create connection 
$conn = mysqli_connect($servername, $username, $password); 

// Check connection 
if (!$conn) { 
    die("Connection failed: " . mysqli_connect_error()); 
} 
echo "Connected successfully\n";

Now, I am getting {"outcome":false,"message":"SQLSTATE[HY000] [2002] Can't connect to local MySQL server through socket '\/var\/lib\/mysql\/mysql.sock' (2)"} while trying t connect through PDO. My attempt through mysql is successful. Seems like the problem is specifically with PDO. Is there anything I can check ?

This is a very old question, but I had the exact same issue and wanted to document it here for anyone who finds this later.

The Problem