Magento 2: How to get order information using REST API?

For order information:

$userData = array("username" => "admin", "password" => "admin123");
$ch = curl_init("http://magento213/index.php/rest/V1/integration/admin/token");
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($userData));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/json", "Content-Lenght: " . strlen(json_encode($userData))));

$token = curl_exec($ch);

$ch = curl_init("http://magento213/index.php/rest/V1/orders/1");
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "GET");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/json", "Authorization: Bearer " . json_decode($token)));

$result = curl_exec($ch);

$result = json_decode($result, 1);
echo '<pre>';print_r($result);

Api List

fetch all pending orders through REST API, add comments on it and change status for those orders

$userData = array("username" => "adminuser", "password" => "adminpassowrd");
$ch = curl_init("https://yourhost.com/index.php/rest/V1/integration/admin/token");
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($userData));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/json", "Content-Lenght: " . strlen(json_encode($userData))));

$token = curl_exec($ch);

$apiOrderUrl = 'https://yourhost.com/index.php/rest/V1/orders/?searchCriteria[filterGroups][0][filters][0][field]=status&searchCriteria[filterGroups][0][filters][0][value]=pending&searchCriteria[filterGroups][0][filters][0][conditionType]=eq';
$ch = curl_init($apiOrderUrl);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "GET");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/json", "Authorization: Bearer " . json_decode($token)));

$result = curl_exec($ch);
$results = json_decode($result, true); // all orders with status pending

foreach ($results['items'] as $order) {
     $commentData = array(
        'id' => $order['entity_id'], //order_id
        'statusHistory' => array(
            'comment' => 'Received order?',
            'entity_id' => null,
            'is_customer_notified' => '1',
            'created_at' => now(),
            'parent_id' => $order['entity_id'], //order_id
            'entity_name' => 'order',
            'status' => 'processing', //assign new status to order
            'is_visible_on_front' => '1'
        )
    );

    $commentData = json_encode($commentData,true);
    $orderStatusApiUrl = 'https://yourhost.com/index.php/rest/V1/orders/'.$order['entity_id'].'/comments';
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL,$orderStatusApiUrl);
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $commentData );
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/json", "Authorization: Bearer " . json_decode($token)));
    $response = curl_exec($ch); //true
}

Go to http://devdocs.magento.com/swagger/

salesOrderRepositoryV1 and customerCustomerRepositoryV1

• Order info: GET: '/V1/orders/{id}'

• Customer info: GET: '/V1/customers/{customerId}'

We can read more here: https://magento.stackexchange.com/a/149927/33057