diff --git a/Tests/Recurly/AccountAcquisition_Test.php b/Tests/Recurly/AccountAcquisition_Test.php
index c568ba96..bf74e7a9 100644
--- a/Tests/Recurly/AccountAcquisition_Test.php
+++ b/Tests/Recurly/AccountAcquisition_Test.php
@@ -2,7 +2,7 @@
 
 class Recurly_AccountAcquisitionTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/accounts/abcdef1234567890/acquisition', 'account_acquisition/show-200.xml'),
       array('PUT', '/accounts/abcdef1234567890/acquisition', 'account_acquisition/create-201.xml'),
diff --git a/Tests/Recurly/AccountBalance_Test.php b/Tests/Recurly/AccountBalance_Test.php
index fc696d14..01c9e46c 100644
--- a/Tests/Recurly/AccountBalance_Test.php
+++ b/Tests/Recurly/AccountBalance_Test.php
@@ -2,7 +2,7 @@
 
 class Recurly_AccountBalanceTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/accounts/abcdef1234567890/balance', 'balance/show-200.xml'),
     );
diff --git a/Tests/Recurly/Account_List_Test.php b/Tests/Recurly/Account_List_Test.php
index 60b54808..cfda8c7a 100644
--- a/Tests/Recurly/Account_List_Test.php
+++ b/Tests/Recurly/Account_List_Test.php
@@ -3,7 +3,7 @@
 
 class Recurly_AccountListTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/accounts', 'accounts/index-200.xml')
     );
diff --git a/Tests/Recurly/Account_Test.php b/Tests/Recurly/Account_Test.php
index d8da3a00..a5e75924 100644
--- a/Tests/Recurly/Account_Test.php
+++ b/Tests/Recurly/Account_Test.php
@@ -3,7 +3,7 @@
 
 class Recurly_AccountTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/accounts/abcdef1234567890', 'accounts/show-200.xml')
     );
diff --git a/Tests/Recurly/Addon_Test.php b/Tests/Recurly/Addon_Test.php
index 8bd990fd..23379529 100644
--- a/Tests/Recurly/Addon_Test.php
+++ b/Tests/Recurly/Addon_Test.php
@@ -3,7 +3,7 @@
 
 class Recurly_AddonTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/plans/gold/add_ons/ipaddresses', 'addons/show-200.xml')
     );
@@ -60,8 +60,8 @@ public function testCreateXmlTieredAddOn() {
     $tier1->ending_quantity = 800;
     $tier2 = new Recurly_Tier();
     $tier2->unit_amount_in_cents->addCurrency('USD', 200);
-  
-    $addon->tiers = array($tier1, $tier2);  
+
+    $addon->tiers = array($tier1, $tier2);
 
     $this->assertXmlStringEqualsXmlString("
       <add_on>
diff --git a/Tests/Recurly/Adjustment_Test.php b/Tests/Recurly/Adjustment_Test.php
index 654c27b8..2e280753 100644
--- a/Tests/Recurly/Adjustment_Test.php
+++ b/Tests/Recurly/Adjustment_Test.php
@@ -3,7 +3,7 @@
 
 class Recurly_AdjustmentTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/adjustments/abcdef1234567890', 'adjustments/show-200.xml')
     );
diff --git a/Tests/Recurly/Base_Test.php b/Tests/Recurly/Base_Test.php
index 4f21e450..ef1402de 100644
--- a/Tests/Recurly/Base_Test.php
+++ b/Tests/Recurly/Base_Test.php
@@ -51,26 +51,26 @@ public function testCheckIfResponseContainGetHeadersFunction() {
     $this->client->addResponse('GET', 'accounts', 'accounts/index-200.xml');
     $accounts = Recurly_Base::_get('accounts', $this->client);
     $this->assertTrue(method_exists($accounts, 'getHeaders'),"Accounts Class does not have method getHeaders");
-    $this->assertInternalType('array', $accounts->getHeaders());
+    $this->assertIsArray($accounts->getHeaders());
 
     $this->client->addResponse('GET', 'subscriptions', 'subscriptions/index-200.xml');
     $subscriptions = Recurly_Base::_get('subscriptions', $this->client);
     $this->assertTrue(method_exists($subscriptions, 'getHeaders'),'Subscriptions Class does not have method getHeaders');
-    $this->assertInternalType('array', $subscriptions->getHeaders());
+    $this->assertIsArray($subscriptions->getHeaders());
 
     $this->client->addResponse('GET', 'abcdef1234567890', 'adjustments/show-200.xml');
     $adjustment = Recurly_Base::_get('abcdef1234567890', $this->client);
     $this->assertTrue(method_exists($adjustment, 'getHeaders'),'Adjustments Class does not have method getHeaders');
-    $this->assertInternalType('array', $adjustment->getHeaders());
+    $this->assertIsArray($adjustment->getHeaders());
   }
 
   public function testPassingEmptyResourceCode() {
     $this->expectException(Recurly_Error::class);
     $uri = Recurly_Base::_safeUri(
-      Recurly_Client::PATH_SUBSCRIPTIONS, "", 
+      Recurly_Client::PATH_SUBSCRIPTIONS, "",
       Recurly_Client::PATH_ADDONS, "marketing_emails",
       Recurly_Client::PATH_USAGE, 123456
-    );    
+    );
   }
 
   public function testUrlEncodingReplacement() {
diff --git a/Tests/Recurly/Billing_Info_Test.php b/Tests/Recurly/Billing_Info_Test.php
index b053fbbe..e7e9374d 100644
--- a/Tests/Recurly/Billing_Info_Test.php
+++ b/Tests/Recurly/Billing_Info_Test.php
@@ -3,7 +3,7 @@
 
 class Recurly_BillingInfoTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/accounts/venmo1234567890/billing_info', 'billing_info/show-venmo-200.xml'),
       array('GET', '/accounts/abcdef1234567890/billing_info', 'billing_info/show-200.xml'),
@@ -117,7 +117,7 @@ public function testGetBecsBillingInfo() {
   public function testVerifyBillingInfoCreditCard() {
     $billing_info = Recurly_BillingInfo::get('abcdef1234567890', $this->client);
     $this->client->addResponse('POST', 'https://api.recurly.com/v2/accounts/abcdef1234567890/billing_info/verify', 'billing_info/verify-200.xml');
-    
+
     $verified = $billing_info->verify();
     $this->assertEquals($verified->origin, 'api_verify_card');
 
@@ -128,7 +128,7 @@ public function testVerifyBillingInfoCreditCard() {
   public function testVerifyCvvBillingInfoCreditCard() {
     $billing_info = Recurly_BillingInfo::get('abcdef1234567890', $this->client);
     $this->client->addResponse('POST', 'https://api.recurly.com/v2/accounts/abcdef1234567890/billing_info/verify_cvv', 'billing_info/verify-cvv-200.xml');
-    
+
     $verified = $billing_info->verifyCvv('988');
     $this->assertInstanceOf('Recurly_BillingInfo', $billing_info);
     $this->assertEquals($verified->year, 2015);
diff --git a/Tests/Recurly/Coupon_Redemption_Test.php b/Tests/Recurly/Coupon_Redemption_Test.php
index 550a0e3b..e62ee0ff 100644
--- a/Tests/Recurly/Coupon_Redemption_Test.php
+++ b/Tests/Recurly/Coupon_Redemption_Test.php
@@ -3,7 +3,7 @@
 
 class Recurly_CouponRedemptionTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/accounts/abcdef1234567890/redemption', 'accounts/redemption/show-200.xml')
     );
diff --git a/Tests/Recurly/Coupon_Test.php b/Tests/Recurly/Coupon_Test.php
index 0628439c..3f679a5b 100644
--- a/Tests/Recurly/Coupon_Test.php
+++ b/Tests/Recurly/Coupon_Test.php
@@ -2,7 +2,7 @@
 
 class Recurly_CouponTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/coupons/special', 'coupons/show-200.xml')
     );
diff --git a/Tests/Recurly/CreditPaymentList_Test.php b/Tests/Recurly/CreditPaymentList_Test.php
index 3c09f297..11c02d3c 100644
--- a/Tests/Recurly/CreditPaymentList_Test.php
+++ b/Tests/Recurly/CreditPaymentList_Test.php
@@ -2,7 +2,7 @@
 
 class Recurly_CreditPaymentListTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/credit_payments', 'credit_payments/index-200.xml')
     );
diff --git a/Tests/Recurly/DunningCampaign_List_Test.php b/Tests/Recurly/DunningCampaign_List_Test.php
index 14eac02d..21b80dc9 100644
--- a/Tests/Recurly/DunningCampaign_List_Test.php
+++ b/Tests/Recurly/DunningCampaign_List_Test.php
@@ -2,7 +2,7 @@
 
 class RecurlyDunningCampaignListTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/dunning_campaigns', 'dunning_campaigns/index-200.xml')
     );
diff --git a/Tests/Recurly/DunningCampaign_Test.php b/Tests/Recurly/DunningCampaign_Test.php
index 9b1bfb01..1f5b1d09 100644
--- a/Tests/Recurly/DunningCampaign_Test.php
+++ b/Tests/Recurly/DunningCampaign_Test.php
@@ -2,7 +2,7 @@
 
 class Recurly_DunningCampaignTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/dunning_campaigns/1234abcd', 'dunning_campaigns/show-200.xml'),
     );
diff --git a/Tests/Recurly/Entitlement_List_Test.php b/Tests/Recurly/Entitlement_List_Test.php
index a378f985..ff8dff46 100644
--- a/Tests/Recurly/Entitlement_List_Test.php
+++ b/Tests/Recurly/Entitlement_List_Test.php
@@ -2,7 +2,7 @@
 
 class RecurlyEntitlementListTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/accounts/abcdef1234567890/entitlements', 'accounts/entitlements/index-200.xml')
     );
diff --git a/Tests/Recurly/ExportDate_List_Test.php b/Tests/Recurly/ExportDate_List_Test.php
index 484354e1..90c56f19 100644
--- a/Tests/Recurly/ExportDate_List_Test.php
+++ b/Tests/Recurly/ExportDate_List_Test.php
@@ -2,7 +2,7 @@
 
 class Recurly_ExportDateList_Test extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/export_dates', 'export_dates/index-200.xml'),
     );
diff --git a/Tests/Recurly/ExportFile_List_Test.php b/Tests/Recurly/ExportFile_List_Test.php
index 43d35d6f..2701fe26 100644
--- a/Tests/Recurly/ExportFile_List_Test.php
+++ b/Tests/Recurly/ExportFile_List_Test.php
@@ -2,7 +2,7 @@
 
 class Recurly_ExportFileList_Test extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/export_dates/2016-08-01/export_files', 'export_files/index-200.xml'),
     );
diff --git a/Tests/Recurly/ExportFile_Test.php b/Tests/Recurly/ExportFile_Test.php
index 20d30fbd..a5b2deeb 100644
--- a/Tests/Recurly/ExportFile_Test.php
+++ b/Tests/Recurly/ExportFile_Test.php
@@ -2,7 +2,7 @@
 
 class Recurly_ExportFile_Test extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/export_dates/2016-08-01/export_files/revenue_schedules_full.csv', 'export_files/show-200.xml'),
     );
diff --git a/Tests/Recurly/GiftCard_List_Test.php b/Tests/Recurly/GiftCard_List_Test.php
index 839a4490..fba07728 100644
--- a/Tests/Recurly/GiftCard_List_Test.php
+++ b/Tests/Recurly/GiftCard_List_Test.php
@@ -2,7 +2,7 @@
 
 class RecurlyGiftCardListTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/gift_cards', 'gift_cards/index-200.xml')
     );
diff --git a/Tests/Recurly/GiftCard_Test.php b/Tests/Recurly/GiftCard_Test.php
index 6042e0f8..cebf85ff 100644
--- a/Tests/Recurly/GiftCard_Test.php
+++ b/Tests/Recurly/GiftCard_Test.php
@@ -2,7 +2,7 @@
 
 class Recurly_GiftCardTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/gift_cards/1988596967980562362', 'gift_cards/show-200.xml')
     );
diff --git a/Tests/Recurly/InvoiceTemplate_List_Test.php b/Tests/Recurly/InvoiceTemplate_List_Test.php
index 226f1fdf..a7c707ba 100644
--- a/Tests/Recurly/InvoiceTemplate_List_Test.php
+++ b/Tests/Recurly/InvoiceTemplate_List_Test.php
@@ -2,7 +2,7 @@
 
 class RecurlyInvoiceTemplateListTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/invoice_templates', 'invoice_templates/index-200.xml')
     );
diff --git a/Tests/Recurly/InvoiceTemplate_Test.php b/Tests/Recurly/InvoiceTemplate_Test.php
index e0e6a70f..bba2972e 100644
--- a/Tests/Recurly/InvoiceTemplate_Test.php
+++ b/Tests/Recurly/InvoiceTemplate_Test.php
@@ -2,7 +2,7 @@
 
 class Recurly_InvoiceTemplateTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/invoice_templates/q0tzf7o7fpbl', 'invoice_templates/show-200.xml'),
     );
diff --git a/Tests/Recurly/Invoice_Test.php b/Tests/Recurly/Invoice_Test.php
index 20f82f45..37159f65 100644
--- a/Tests/Recurly/Invoice_Test.php
+++ b/Tests/Recurly/Invoice_Test.php
@@ -4,7 +4,7 @@
 class Recurly_InvoiceTest extends Recurly_TestCase
 {
 
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/invoices/1001', 'invoices/show-200.xml'),
       array('GET', '/invoices/1002', 'invoices/show-with-prefix-200.xml'),
diff --git a/Tests/Recurly/Item_Test.php b/Tests/Recurly/Item_Test.php
index b0931ffa..ce6b3352 100644
--- a/Tests/Recurly/Item_Test.php
+++ b/Tests/Recurly/Item_Test.php
@@ -2,7 +2,7 @@
 
 class Recurly_ItemTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/items/plastic_gloves', 'items/show-200.xml'),
     );
@@ -17,7 +17,7 @@ public function testConstructor() {
 
     $this->assertSame($client, $prop->getValue($item));
   }
-  
+
   public function testGetItem() {
     $item = Recurly_Item::get('plastic_gloves', $this->client);
 
diff --git a/Tests/Recurly/Plan_Test.php b/Tests/Recurly/Plan_Test.php
index 01b8e323..c5e48719 100644
--- a/Tests/Recurly/Plan_Test.php
+++ b/Tests/Recurly/Plan_Test.php
@@ -3,7 +3,7 @@
 
 class Recurly_PlanTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/plans/silver', 'plans/show-200.xml'),
       array('GET', '/plans/ramp-priced-plan', 'plans/show-ramps-200.xml'),
diff --git a/Tests/Recurly/Purchase_Test.php b/Tests/Recurly/Purchase_Test.php
index fafb7ec4..f450c65f 100644
--- a/Tests/Recurly/Purchase_Test.php
+++ b/Tests/Recurly/Purchase_Test.php
@@ -2,7 +2,7 @@
 
 class Recurly_PurchaseTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('POST', '/purchases', 'purchases/create-201.xml'),
       array('POST', '/purchases/preview', 'purchases/preview-200.xml'),
diff --git a/Tests/Recurly/ShippingAddress_Test.php b/Tests/Recurly/ShippingAddress_Test.php
index cd6d9e73..5df16e46 100644
--- a/Tests/Recurly/ShippingAddress_Test.php
+++ b/Tests/Recurly/ShippingAddress_Test.php
@@ -2,7 +2,7 @@
 
 class Recurly_ShippingAddressTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/accounts/abcdef1234567890', 'accounts/show-200.xml')
     );
diff --git a/Tests/Recurly/Transaction_Test.php b/Tests/Recurly/Transaction_Test.php
index 322620d6..cefd77e1 100644
--- a/Tests/Recurly/Transaction_Test.php
+++ b/Tests/Recurly/Transaction_Test.php
@@ -3,7 +3,7 @@
 
 class Recurly_TransactionTest extends Recurly_TestCase
 {
-  function defaultResponses() {
+  function defaultResponses(): array {
     return array(
       array('GET', '/transactions/012345678901234567890123456789ab', 'transactions/show-200.xml'),
       array('GET', '/invoices/1001', 'invoices/show-200.xml'),
diff --git a/Tests/bootstrap.php b/Tests/bootstrap.php
index e31eb9ce..a9a578df 100644
--- a/Tests/bootstrap.php
+++ b/Tests/bootstrap.php
@@ -13,11 +13,10 @@
 
 /**
  * Base class for our tests that sets up a mock client.
- *
- * @property Recurly_MockClient $client
  */
 abstract class Recurly_TestCase extends TestCase {
-  function setUp() {
+  protected Recurly_MockClient $client;
+  function setUp(): void {
     $this->client = new Recurly_MockClient();
     foreach ($this->defaultResponses() as $request) {
       call_user_func_array(array($this->client, 'addResponse'), $request);
@@ -27,7 +26,8 @@ function setUp() {
   /**
    * Return an array of responses that will be added to the mock client.
    */
-  function defaultResponses() {
+  function defaultResponses(): array
+  {
     return array();
   }
 }
@@ -36,18 +36,22 @@ function defaultResponses() {
  * Return canned client responses.
  */
 class Recurly_MockClient {
+  private array $_responses;
+
   public function __construct() {
     $this->_responses = array();
   }
 
-  public function addResponse($method, $uri, $fixture_filename) {
+  public function addResponse($method, $uri, $fixture_filename): void
+  {
     if (!isset($this->_responses[$method])) {
       $this->_responses[$method] = array();
     }
     $this->_responses[$method][$uri] = $fixture_filename;
   }
 
-  public function request($method, $uri, $data = null) {
+  public function request($method, $uri, $data = null): Recurly_ClientResponse
+  {
     if (isset($this->_responses[$method][$uri])) {
       $fixture_filename = $this->_responses[$method][$uri];
     }
@@ -58,11 +62,13 @@ public function request($method, $uri, $data = null) {
     return $this->responseFromFixture($fixture_filename);
   }
 
-  public function getPdf($uri, $locale = null) {
+  public function getPdf($uri, $locale = null): array
+  {
     return array($uri, $locale);
   }
 
-  protected function responseFromFixture($filename) {
+  protected function responseFromFixture($filename): Recurly_ClientResponse
+  {
     $statusCode = 200;
     $headers = array();
     $body = null;
diff --git a/composer.json b/composer.json
index e1140b12..f9bf0cbd 100644
--- a/composer.json
+++ b/composer.json
@@ -11,7 +11,7 @@
         "ext-curl": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": ">=5.7,<8"
+        "phpunit/phpunit": "9.*"
     },
     "autoload": {
         "classmap": ["lib"]
diff --git a/lib/recurly/client.php b/lib/recurly/client.php
index 92d9d7cf..921fdedc 100644
--- a/lib/recurly/client.php
+++ b/lib/recurly/client.php
@@ -136,7 +136,9 @@ public static function disableXmlEntityLoading()
         $disable = (bool)$disable;
     }
 
-    libxml_disable_entity_loader($disable);
+    if (\PHP_VERSION_ID < 80000) {
+      libxml_disable_entity_loader($disable);
+    }
   }
 
   /**
diff --git a/lib/recurly/coupon.php b/lib/recurly/coupon.php
index 843755f9..24adb89b 100644
--- a/lib/recurly/coupon.php
+++ b/lib/recurly/coupon.php
@@ -122,7 +122,7 @@ public function createUpdateXML() {
         $val = $val->format('c');
       }
 
-      $root->appendChild($doc->createElement($attr, $val));
+      $root->appendChild($doc->createElement($attr, $val ?? ''));
     }
 
     return XmlTools::renderXML($doc);
diff --git a/lib/recurly/currency_list.php b/lib/recurly/currency_list.php
index 246af95d..5985ae89 100644
--- a/lib/recurly/currency_list.php
+++ b/lib/recurly/currency_list.php
@@ -10,7 +10,7 @@ function __construct($nodeName) {
     $this->currencies = array();
   }
 
-  public function addCurrency($currencyCode, $amountInCents) {
+  public function addCurrency($currencyCode, $amountInCents): void {
     if (is_string($currencyCode) && strlen($currencyCode) == 3) {
       $this->currencies[$currencyCode] = new Recurly_Currency($currencyCode, $amountInCents);
     }
@@ -20,18 +20,19 @@ public function getCurrency($currencyCode) {
     return isset($this->currencies[$currencyCode]) ? $this->currencies[$currencyCode] : null;
   }
 
-  public function offsetSet($offset, $value) {
-    return $this->addCurrency($offset, $value);
+  public function offsetSet($offset, $value): void {
+    $this->addCurrency($offset, $value);
   }
 
-  public function offsetExists($offset) {
+  public function offsetExists($offset): bool {
     return isset($this->currencies[$offset]);
   }
 
-  public function offsetUnset($offset) {
+  public function offsetUnset($offset): void {
     unset($this->currencies[$offset]);
   }
 
+  #[ReturnTypeWillChange]
   public function offsetGet($offset) {
     return $this->getCurrency($offset);
   }
@@ -44,11 +45,11 @@ public function __get($k) {
     return $this->offsetGet($k);
   }
 
-  public function count() {
+  public function count(): int {
     return count($this->currencies);
   }
 
-  public function getIterator() {
+  public function getIterator(): Traversable {
     return new ArrayIterator($this->currencies);
   }
 
diff --git a/lib/recurly/custom_field_list.php b/lib/recurly/custom_field_list.php
index 9604d5a7..38944845 100644
--- a/lib/recurly/custom_field_list.php
+++ b/lib/recurly/custom_field_list.php
@@ -10,7 +10,7 @@ class Recurly_CustomFieldList extends ArrayObject
    * @param object $value Must be instance of Recurly_CustomField
    * @throws Exception
    */
-  public function offsetSet($index, $value) {
+  public function offsetSet($index, $value): void {
     if (!$value instanceof Recurly_CustomField) {
       throw new Exception("value must be an instance of Recurly_CustomField");
     }
@@ -25,7 +25,7 @@ public function offsetSet($index, $value) {
     parent::offsetSet($index, $value);
   }
 
-  public function offsetUnset($index) {
+  public function offsetUnset($index): void {
     parent::offsetSet($index, new Recurly_CustomField($index, null));
   }
 
diff --git a/lib/recurly/error_list.php b/lib/recurly/error_list.php
index 8b49822d..2e1c3991 100644
--- a/lib/recurly/error_list.php
+++ b/lib/recurly/error_list.php
@@ -25,29 +25,30 @@ function __construct() {
   }
 
   // array access to the errors collection
-  public function offsetSet($offset, $value) {
+  public function offsetSet($offset, $value): void {
     if (is_null($offset)) {
       $this->errors[] = $value;
     } else {
       $this->errors[$offset] = $value;
     }
   }
-  public function offsetExists($offset) {
+  public function offsetExists($offset): bool {
     return isset($this->errors[$offset]);
   }
-  public function offsetUnset($offset) {
+  public function offsetUnset($offset): void {
     unset($this->errors[$offset]);
   }
+  #[ReturnTypeWillChange]
   public function offsetGet($offset) {
     return isset($this->errors[$offset]) ? $this->errors[$offset] : null;
   }
 
-  public function count()
+  public function count(): int
   {
     return count($this->errors);
   }
 
-  public function getIterator() {
+  public function getIterator(): Traversable {
     return new ArrayIterator($this->errors);
   }
 
diff --git a/lib/recurly/pager.php b/lib/recurly/pager.php
index 8201be59..3aa33bfc 100644
--- a/lib/recurly/pager.php
+++ b/lib/recurly/pager.php
@@ -18,6 +18,7 @@ abstract class Recurly_Pager extends Recurly_Base implements Iterator, Countable
    * @return integer number of records in list
    * @throws Recurly_Error
    */
+  #[ReturnTypeWillChange]
   public function count() {
     if (isset($this->_href)) {
       $headers = Recurly_Base::_head($this->_href, $this->_client);
@@ -47,7 +48,7 @@ protected function get_first_page() {
    *
    * @throws Recurly_Error
    */
-  public function rewind() {
+  public function rewind(): void {
     $this->_loadFrom($this->_href);
     $this->_position = 0;
   }
@@ -58,6 +59,7 @@ public function rewind() {
    * @return Recurly_Resource the current object
    * @throws Recurly_Error
    */
+  #[ReturnTypeWillChange]
   public function current()
   {
     // Work around pre-PHP 5.5 issue that prevents `empty($this->count())`:
@@ -84,6 +86,7 @@ public function current()
   /**
    * @return integer current position within the current page
    */
+  #[ReturnTypeWillChange]
   public function key() {
     return $this->_position;
   }
@@ -91,14 +94,14 @@ public function key() {
   /**
    * Increments the position to the next element
    */
-  public function next() {
+  public function next(): void {
     ++$this->_position;
   }
 
   /**
    * @return boolean True if the current position is valid.
    */
-  public function valid() {
+  public function valid(): bool {
     return (isset($this->_objects[$this->_position]) || isset($this->_links['next']));
   }