Util.php

<?php
namespace Model\Common\Util;

use Aws\Ec2\Ec2Client;

class Ec2Util
{
    private $ec2;
    private $ips = array();

    public function __construct($key, $secret, $region)
    {
        try {
             $this->ec2 = Ec2Client::factory(array(
                 'key' => $key,
                 'secret' => $secret,
                 'region' => $region
             ));
             $response = $this->ec2->describeAddresses();
             if ($response) {
                 $instances = $response->toArray();
                 $address = $instances['Addresses'];
                 foreach ($address as $data) {
                     $this->ips[$data['PrivateIpAddress']] = $data['PublicIp'];
                 }
             }
         } catch (\Exception $e) {
         }
    }

    public function getPublicIp($privateIp)
    {
        if (isset($this->ips[$privateIp])) {
            return $this->ips[$privateIp];
        } else {
            return '';
        }
    }
}

もちろんComposerでSDKをいれておく

composer.json

{
    "require" : {
        "aws/aws-sdk-php" : "2.*"
    }
}

Ec2Utilは適当にインスタンス化して（必要な情報を渡して)
getPublicIpメソッドを呼べばOK

引数のIPはプライベートIP
PrivateなIPを簡単に取得できるはず（$_SERVER[ 'SERVER_ADDR' ]とか？）