cloudfront.php

<?php

require './vendor/autoload.php';

use Aws\CloudFront\CloudFrontClient;

$cloudfront = CloudFrontClient::factory([
    'private_key' => getcwd() . '/private_key.pem',
    'key_pair_id' => 'YYYYYYYYYYYY'
]);

$origin   = 'http://xxxxxxxxxxx.cloudfront.net';
$resource = 'test.jpg';
$expires  = time() + 300;

$signed = $cloudfront->getSignedUrl([
    'url'     => $origin . '/' . $resource,
    'expires' => $expires
]);

echo $signed;

cloudfront.rb

require 'aws_cf_signer'

origin   = 'http://xxxxxxxxxxx.cloudfront.net'
resource = 'test.jpg'

signer = AwsCfSigner.new('/path/to/private_key.pem', 'YYYYYYYYYYYY')
signed = signer.sign("#{origin}/#{resource}", ending: Time.now + 300)

puts signed