diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 13ae07a..a2f6c2c 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -17,7 +17,7 @@ jobs:
         strategy:
             fail-fast: false
             matrix:
-                php: ['8.3', '8.4', '8.5']
+                php: ['8.1', '8.2', '8.3', '8.4', '8.5']
         steps:
             - name: Setup PHP
               uses: shivammathur/setup-php@v2
diff --git a/composer.json b/composer.json
index d13b165..caa34b8 100644
--- a/composer.json
+++ b/composer.json
@@ -4,11 +4,10 @@
     "keywords": ["dictionary", "set", "lookup", "binary", "hash", "memory", "fingerprint"],
     "type": "library",
     "require":  {
-        "php": "^8.3"
+        "php": "^8.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^12.5",
-        "symfony/filesystem": "^7.4 || ^8.0",
+        "phpunit/phpunit": "^10.5 || ^11.0 || ^12.5",
         "terminal42/contao-build-tools": "@dev"
     },
     "license": "MIT",
diff --git a/phpstan.neon b/phpstan.neon
new file mode 100644
index 0000000..43d5e33
--- /dev/null
+++ b/phpstan.neon
@@ -0,0 +1,5 @@
+parameters:
+    ignoreErrors:
+        -
+            identifier: filesystemcall.unsafe
+            path: tests/FastSetTest.php
diff --git a/phpunit.xml b/phpunit.xml
index c2048ac..70f54b7 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -17,7 +17,7 @@
         </testsuite>
     </testsuites>
 
-    <source ignoreIndirectDeprecations="true" restrictNotices="true" restrictWarnings="true">
+    <source restrictNotices="true" restrictWarnings="true">
         <include>
             <directory>src</directory>
         </include>
diff --git a/src/FastSet.php b/src/FastSet.php
index 2874dff..c4dbb33 100644
--- a/src/FastSet.php
+++ b/src/FastSet.php
@@ -12,7 +12,7 @@ final class FastSet
      * With a 16-bit prefix, a lot of datasets end up with tiny buckets most of the time,
      * so avoiding the extra branching of binary search wins in practice.
      */
-    private const int LINEAR_SCAN_THRESHOLD = 8;
+    private const LINEAR_SCAN_THRESHOLD = 8;
 
     private readonly string $hashesPath;
 
diff --git a/tests/FastSetTest.php b/tests/FastSetTest.php
index 6a522a5..3ef6a3d 100644
--- a/tests/FastSetTest.php
+++ b/tests/FastSetTest.php
@@ -6,7 +6,6 @@
 
 use PHPUnit\Framework\Attributes\TestWith;
 use PHPUnit\Framework\TestCase;
-use Symfony\Component\Filesystem\Filesystem;
 use Toflar\FastSet\FastSet;
 use Toflar\FastSet\SetBuilder;
 
@@ -17,9 +16,8 @@ final class FastSetTest extends TestCase
     protected function setUp(): void
     {
         $testDir = __DIR__.'/../var';
-        $fs = new Filesystem();
-        $fs->remove($testDir);
-        $fs->mkdir($testDir);
+        $this->removeDirectory($testDir);
+        mkdir($testDir, 0777, true);
         $this->testDirectory = $testDir;
     }
 
@@ -76,4 +74,22 @@ private function assertFastSetContents(FastSet $fastSet): void
         $this->assertTrue($fastSet->has('aab'));
         $this->assertFalse($fastSet->has('foobar'));
     }
+
+    private function removeDirectory(string $directory): void
+    {
+        if (!is_dir($directory)) {
+            return;
+        }
+
+        $iterator = new \RecursiveIteratorIterator(
+            new \RecursiveDirectoryIterator($directory, \FilesystemIterator::SKIP_DOTS),
+            \RecursiveIteratorIterator::CHILD_FIRST,
+        );
+
+        foreach ($iterator as $file) {
+            $file->isDir() ? rmdir($file->getPathname()) : unlink($file->getPathname());
+        }
+
+        rmdir($directory);
+    }
 }