AWS SDK PHP Class Not Found

When I am use the AWS SDK with php I get an error stating that the class cannot be found.

    <?php
    require_once  '/var/www/html/aws.phar';
    use Aws\Common\Aws;
    use Aws\Common\Enum\Region;
    echo "test";
    $AWSregion = Region::US_EAST_1;

    $aws = Aws\Common\Aws::factory(array('key'=>'key', 'secret' => 'secret_key', 'region' => $AWSregion));
    $client = $aws->get('Sqs');

?>

The exact error message I receive is PHP Fatal Error: Class 'Aws\Common\Enum\Region' not found in /var/www/html/sendSQS.php

Any help is appreciated.

Thanks!