diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
index 2f1fb9f465..694fa99eab 100644
--- a/.github/workflows/lint.yml
+++ b/.github/workflows/lint.yml
@@ -41,7 +41,7 @@ jobs:
           php-version: "${{ matrix.php-version }}"
 
       - name: "Downgrade PHPUnit"
-        if: matrix.php-version == '7.4' || matrix.php-version == '8.0' || matrix.php-version == '8.1'
+        if: matrix.php-version == '7.4' || matrix.php-version == '8.0'
         run: "composer require --dev phpunit/phpunit:^9.6 sebastian/diff:^4.0 --update-with-dependencies --ignore-platform-reqs"
 
       - name: "Install dependencies"
diff --git a/.github/workflows/static-analysis.yml b/.github/workflows/static-analysis.yml
index a49e5e7623..de5d674ca5 100644
--- a/.github/workflows/static-analysis.yml
+++ b/.github/workflows/static-analysis.yml
@@ -50,7 +50,7 @@ jobs:
           extensions: mbstring
 
       - name: "Downgrade PHPUnit"
-        if: matrix.php-version == '7.4' || matrix.php-version == '8.0' || matrix.php-version == '8.1'
+        if: matrix.php-version == '7.4' || matrix.php-version == '8.0'
         shell: bash
         run: "composer require --dev phpunit/phpunit:^9.6 sebastian/diff:^4.0 --update-with-dependencies --ignore-platform-reqs"
 
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index c431ecee52..93b506fcbc 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -32,6 +32,7 @@ jobs:
       fail-fast: false
       matrix:
         php-version:
+          - "8.1"
           - "8.2"
           - "8.3"
           - "8.4"
@@ -55,6 +56,14 @@ jobs:
       - name: "Install dependencies"
         run: "composer install --no-interaction --no-progress"
 
+      - name: "Transform source code"
+        if: matrix.php-version == '8.1'
+        shell: bash
+        run: |
+          composer install --no-interaction --no-progress --working-dir=compiler
+          ./compiler/vendor/bin/simple-downgrade downgrade -c build/downgrade.php ${{ matrix.php-version }}
+          composer dump
+
       - name: "Tests"
         run: "make tests"
 
@@ -159,7 +168,6 @@ jobs:
         php-version:
           - "7.4"
           - "8.0"
-          - "8.1"
         operating-system: [ ubuntu-latest, windows-latest ]
 
     steps:
diff --git a/build/ignore-by-php-version.neon.php b/build/ignore-by-php-version.neon.php
index e84ddbd635..91b74af9f4 100644
--- a/build/ignore-by-php-version.neon.php
+++ b/build/ignore-by-php-version.neon.php
@@ -35,7 +35,7 @@
 	$includes[] = __DIR__ . '/deprecated-8.4.neon';
 }
 
-if (PHP_VERSION_ID < 80200) {
+if (PHP_VERSION_ID < 80100) {
 	$includes[] = __DIR__ . '/old-phpunit.neon';
 } else {
 	$includes[] = __DIR__ . '/new-phpunit.neon';