Amazon AWS SNS register application

Im trying to register an application using AWS SDK with the following code

$AmazonSNS = SnsClient::factory(array(
'key'    => $sns_id,
'secret' => $sns_secret,
'region' => 'us-east-1'

));

$app_details = array(
    'Name' => $app_name,
    'Platform' => 'APNS',
    'Attributes' => array(
        'PlatformCredential ' => $pem, //PEM IS A STRING
        'PlatformPrincipal' => ''
    )
);

$results = $AmazonSNS->createPlatformApplication($app_details);

All I get is the following exception

<b>Fatal error</b>:  Uncaught Aws\Sns\Exception\InvalidParameterException: AWS Error Code: InvalidParameter, Status Code: 400, AWS Request ID: 059d5491-75a5-5f7b-9183-190339511e06, AWS Error Type: client, AWS Error Message: Invalid parameter: Attributes Reason: Invalid attribute name: PlatformCredential , User-Agent: aws-sdk-php2/2.7.25 Guzzle/3.9.3 curl/7.38.0 PHP/5.5.22
  thrown in <b>/home/notifications/public_html/Aws/Aws/Common/Exception/NamespaceExceptionFactory.php</b> on line <b>91</b><br />

My goal is to register an application to AWS for future use on sending notifications to the app users.

What is wrong with my code ?