diff --git a/.travis.yml b/.travis.yml
index 5dd52b3..3e3177f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,5 +5,4 @@ import:
 
 env:
   global:
-    - COMPOSER_ROOT_VERSION="2.x-dev"
     - BEHAT_SUITE="@subsites"
diff --git a/composer.json b/composer.json
index 91cda69..eeca054 100644
--- a/composer.json
+++ b/composer.json
@@ -15,7 +15,8 @@
         }
     ],
     "require": {
-        "silverstripe/framework": "^4.4@dev",
+        "php": "^7.3 || ^8.0",
+        "silverstripe/framework": "^4.10",
         "silverstripe/cms": "^4.4@dev",
         "silverstripe/admin": "^1.4@dev",
         "silverstripe/asset-admin": "^1.4@dev",
@@ -23,7 +24,7 @@
         "silverstripe/versioned": "^1.4@dev"
     },
     "require-dev": {
-        "sminnee/phpunit": "^5.7",
+        "phpunit/phpunit": "^9.5",
         "squizlabs/php_codesniffer": "^3.0"
     },
     "autoload": {
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 8b43ffd..6a634a2 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,7 +1,9 @@
 <phpunit bootstrap="vendor/silverstripe/cms/tests/bootstrap.php" colors="true">
-    <testsuite name="Default">
-        <directory>tests/php</directory>
-    </testsuite>
+    <testsuites>
+        <testsuite name="Default">
+            <directory>tests/php</directory>
+        </testsuite>
+    </testsuites>
 
     <filter>
         <whitelist addUncoveredFilesFromWhitelist="true">
diff --git a/tests/php/BaseSubsiteTest.php b/tests/php/BaseSubsiteTest.php
index 705a31f..ca29c68 100644
--- a/tests/php/BaseSubsiteTest.php
+++ b/tests/php/BaseSubsiteTest.php
@@ -9,7 +9,7 @@ use SilverStripe\Subsites\State\SubsiteState;
 
 class BaseSubsiteTest extends SapphireTest
 {
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();
 
diff --git a/tests/php/GroupSubsitesTest.php b/tests/php/GroupSubsitesTest.php
index c2eaa3e..5a0b19e 100644
--- a/tests/php/GroupSubsitesTest.php
+++ b/tests/php/GroupSubsitesTest.php
@@ -15,8 +15,8 @@ class GroupSubsitesTest extends BaseSubsiteTest
 
     public function testTrivialFeatures()
     {
-        $this->assertInternalType('array', singleton(GroupSubsites::class)->extraStatics());
-        $this->assertInternalType('array', singleton(GroupSubsites::class)->providePermissions());
+        $this->assertIsArray(singleton(GroupSubsites::class)->extraStatics());
+        $this->assertIsArray(singleton(GroupSubsites::class)->providePermissions());
         $this->assertInstanceOf(FieldList::class, singleton(Group::class)->getCMSFields());
     }
 
diff --git a/tests/php/InitStateMiddlewareTest.php b/tests/php/InitStateMiddlewareTest.php
index 5476f72..4ed6280 100644
--- a/tests/php/InitStateMiddlewareTest.php
+++ b/tests/php/InitStateMiddlewareTest.php
@@ -19,14 +19,14 @@ class InitStateMiddlewareTest extends BaseSubsiteTest
      */
     protected $origServer = [];
 
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();
 
         $this->origServer = $_SERVER;
     }
 
-    protected function tearDown()
+    protected function tearDown(): void
     {
         $_SERVER = $this->origServer;
 
diff --git a/tests/php/Service/ThemeResolverTest.php b/tests/php/Service/ThemeResolverTest.php
index 0b98f3b..7f0fd7e 100644
--- a/tests/php/Service/ThemeResolverTest.php
+++ b/tests/php/Service/ThemeResolverTest.php
@@ -18,7 +18,7 @@ class ThemeResolverTest extends SapphireTest
         SSViewer::DEFAULT_THEME,
     ];
 
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();
 
diff --git a/tests/php/SiteTreeSubsitesTest.php b/tests/php/SiteTreeSubsitesTest.php
index 9adb321..4a9c667 100644
--- a/tests/php/SiteTreeSubsitesTest.php
+++ b/tests/php/SiteTreeSubsitesTest.php
@@ -42,7 +42,7 @@ class SiteTreeSubsitesTest extends BaseSubsiteTest
         ],
     ];
 
-    protected function setUp()
+    protected function setUp(): void
     {
         // We have our own home page fixtures, prevent the default one being created in this test suite.
         Config::modify()->set(SiteTree::class, 'create_default_pages', false);
diff --git a/tests/php/SubsiteAdminFunctionalTest.php b/tests/php/SubsiteAdminFunctionalTest.php
index 029ed26..e2993b0 100644
--- a/tests/php/SubsiteAdminFunctionalTest.php
+++ b/tests/php/SubsiteAdminFunctionalTest.php
@@ -14,7 +14,7 @@ class SubsiteAdminFunctionalTest extends FunctionalTest
 
     protected $autoFollowRedirection = false;
 
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();
         // Ensure all pages are published
@@ -48,14 +48,18 @@ class SubsiteAdminFunctionalTest extends FunctionalTest
         $this->logOut();
 
         $response = $this->getAndFollowAll('admin/pages/?SubsiteID=0');
-        $this->assertContains('Security/login', $this->mainSession->lastUrl(), 'Admin is disallowed');
+        $this->assertStringContainsString('Security/login', $this->mainSession->lastUrl(), 'Admin is disallowed');
 
         $subsite1 = $this->objFromFixture(Subsite::class, 'subsite1');
         $response = $this->getAndFollowAll("admin/pages/?SubsiteID={$subsite1->ID}");
-        $this->assertContains('Security/login', $this->mainSession->lastUrl(), 'Admin is disallowed');
+        $this->assertStringContainsString('Security/login', $this->mainSession->lastUrl(), 'Admin is disallowed');
 
         $response = $this->getAndFollowAll('admin/subsite_xhr');
-        $this->assertContains('Security/login', $this->mainSession->lastUrl(), 'SubsiteXHRController is disallowed');
+        $this->assertStringContainsString(
+            'Security/login',
+            $this->mainSession->lastUrl(),
+            'SubsiteXHRController is disallowed'
+        );
     }
 
     /**
@@ -67,17 +71,21 @@ class SubsiteAdminFunctionalTest extends FunctionalTest
 
         $this->getAndFollowAll('admin/pages/?SubsiteID=0');
         $this->assertEquals(0, $this->session()->get('SubsiteID'), 'Can access main site.');
-        $this->assertContains('admin/pages', $this->mainSession->lastUrl(), 'Lands on the correct section');
+        $this->assertStringContainsString('admin/pages', $this->mainSession->lastUrl(), 'Lands on the correct section');
 
         $subsite1 = $this->objFromFixture(Subsite::class, 'subsite1');
         $this->getAndFollowAll("admin/pages/?SubsiteID={$subsite1->ID}");
 
         // Check the session manually, since the state is unique to the request, not this test
         $this->assertEquals($subsite1->ID, $this->session()->get('SubsiteID'), 'Can access other subsite.');
-        $this->assertContains('admin/pages', $this->mainSession->lastUrl(), 'Lands on the correct section');
+        $this->assertStringContainsString('admin/pages', $this->mainSession->lastUrl(), 'Lands on the correct section');
 
         $response = $this->getAndFollowAll('admin/subsite_xhr');
-        $this->assertNotContains('Security/login', $this->mainSession->lastUrl(), 'SubsiteXHRController is reachable');
+        $this->assertStringNotContainsString(
+            'Security/login',
+            $this->mainSession->lastUrl(),
+            'SubsiteXHRController is reachable'
+        );
     }
 
     public function testAdminIsRedirectedToObjectsSubsite()
@@ -92,7 +100,7 @@ class SubsiteAdminFunctionalTest extends FunctionalTest
         $response = $this->get("admin/pages/edit/show/$subsite1Home->ID");
 
         $this->assertEquals(302, $response->getStatusCode());
-        $this->assertContains(
+        $this->assertStringContainsString(
             'admin/pages/edit/show/' . $subsite1Home->ID . '?SubsiteID=' . $subsite1Home->SubsiteID,
             $response->getHeader('Location')
         );
@@ -102,7 +110,7 @@ class SubsiteAdminFunctionalTest extends FunctionalTest
 
         $response = $this->get("admin/pages/edit/show/$subsite1Home->ID");
         $this->assertEquals(302, $response->getStatusCode());
-        $this->assertContains(
+        $this->assertStringContainsString(
             'admin/pages/edit/show/' . $subsite1Home->ID . '?SubsiteID=' . $subsite1Home->SubsiteID,
             $response->getHeader('Location')
         );
@@ -122,15 +130,19 @@ class SubsiteAdminFunctionalTest extends FunctionalTest
 
         $this->get('admin/pages/?SubsiteID=0');
         $this->assertEquals(0, $this->session()->get('SubsiteID'), 'Can access main site.');
-        $this->assertContains('admin/pages', $this->mainSession->lastUrl(), 'Lands on the correct section');
+        $this->assertStringContainsString('admin/pages', $this->mainSession->lastUrl(), 'Lands on the correct section');
 
         $subsite1 = $this->objFromFixture(Subsite::class, 'subsite1');
         $this->get("admin/pages/?SubsiteID={$subsite1->ID}");
         $this->assertEquals($subsite1->ID, $this->session()->get('SubsiteID'), 'Can access other subsite.');
-        $this->assertContains('admin/pages', $this->mainSession->lastUrl(), 'Lands on the correct section');
+        $this->assertStringContainsString('admin/pages', $this->mainSession->lastUrl(), 'Lands on the correct section');
 
         $response = $this->get('admin/subsite_xhr');
-        $this->assertNotContains('Security/login', $this->mainSession->lastUrl(), 'SubsiteXHRController is reachable');
+        $this->assertStringNotContainsString(
+            'Security/login',
+            $this->mainSession->lastUrl(),
+            'SubsiteXHRController is reachable'
+        );
     }
 
     /**
@@ -146,7 +158,11 @@ class SubsiteAdminFunctionalTest extends FunctionalTest
         // Check allowed URL.
         $this->getAndFollowAll("admin/pages/?SubsiteID={$subsite1->ID}");
         $this->assertEquals($subsite1->ID, $this->session()->get('SubsiteID'), 'Can access own subsite.');
-        $this->assertContains('admin/pages', $this->mainSession->lastUrl(), 'Can access permitted section.');
+        $this->assertStringContainsString(
+            'admin/pages',
+            $this->mainSession->lastUrl(),
+            'Can access permitted section.'
+        );
 
         // Check forbidden section in allowed subsite.
         $this->getAndFollowAll("admin/assets/?SubsiteID={$subsite1->ID}");
@@ -172,10 +188,14 @@ class SubsiteAdminFunctionalTest extends FunctionalTest
             $subsite1->ID,
             'Is redirected to first permitted subsite.'
         );
-        $this->assertNotContains('Security/login', $this->mainSession->lastUrl(), 'Is not denied access');
+        $this->assertStringNotContainsString('Security/login', $this->mainSession->lastUrl(), 'Is not denied access');
 
         // Check the standalone XHR controller.
         $response = $this->getAndFollowAll('admin/subsite_xhr');
-        $this->assertNotContains('Security/login', $this->mainSession->lastUrl(), 'SubsiteXHRController is reachable');
+        $this->assertStringNotContainsString(
+            'Security/login',
+            $this->mainSession->lastUrl(),
+            'SubsiteXHRController is reachable'
+        );
     }
 }
diff --git a/tests/php/SubsiteAdminTest.php b/tests/php/SubsiteAdminTest.php
index 9e3a282..0fc10a8 100644
--- a/tests/php/SubsiteAdminTest.php
+++ b/tests/php/SubsiteAdminTest.php
@@ -11,7 +11,7 @@ class SubsiteAdminTest extends FunctionalTest
 {
     protected static $fixture_file = 'SubsiteTest.yml';
 
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();
 
diff --git a/tests/php/SubsiteTest.php b/tests/php/SubsiteTest.php
index fdd0670..1531315 100644
--- a/tests/php/SubsiteTest.php
+++ b/tests/php/SubsiteTest.php
@@ -26,7 +26,7 @@ class SubsiteTest extends BaseSubsiteTest
      */
     protected $origServer = [];
 
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();
 
@@ -38,7 +38,7 @@ class SubsiteTest extends BaseSubsiteTest
         $this->origServer = $_SERVER;
     }
 
-    protected function tearDown()
+    protected function tearDown(): void
     {
         $_SERVER = $this->origServer;
 
diff --git a/tests/php/SubsiteXHRControllerTest.php b/tests/php/SubsiteXHRControllerTest.php
index 5ec92fb..521df92 100644
--- a/tests/php/SubsiteXHRControllerTest.php
+++ b/tests/php/SubsiteXHRControllerTest.php
@@ -36,12 +36,12 @@ class SubsiteXHRControllerTest extends FunctionalTest
 
         $this->assertEquals(200, $result->getStatusCode());
         // SilverStripe 4.0-4.2: text/json. >=4.3: application/json
-        $this->assertContains('json', $result->getHeader('Content-Type'));
+        $this->assertStringContainsString('json', $result->getHeader('Content-Type'));
 
         $body = $result->getBody();
-        $this->assertContains('Main site', $body);
-        $this->assertContains('Test 1', $body);
-        $this->assertContains('Test 2', $body);
-        $this->assertContains('Test 3', $body);
+        $this->assertStringContainsString('Main site', $body);
+        $this->assertStringContainsString('Test 1', $body);
+        $this->assertStringContainsString('Test 2', $body);
+        $this->assertStringContainsString('Test 3', $body);
     }
 }
diff --git a/tests/php/SubsitesVirtualPageTest.php b/tests/php/SubsitesVirtualPageTest.php
index 527927f..af98f4b 100644
--- a/tests/php/SubsitesVirtualPageTest.php
+++ b/tests/php/SubsitesVirtualPageTest.php
@@ -21,7 +21,7 @@ class SubsitesVirtualPageTest extends BaseSubsiteTest
         'SubsitesVirtualPageTest.yml',
     ];
 
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();
 
@@ -43,7 +43,7 @@ class SubsitesVirtualPageTest extends BaseSubsiteTest
         $page->write();
     }
 
-    public function tearDown()
+    protected function tearDown(): void
     {
         TestAssetStore::reset();
         parent::tearDown();