diff --git a/.github/workflows/php.yml b/.github/workflows/php.yml
index 1b2d09a..6b682c4 100644
--- a/.github/workflows/php.yml
+++ b/.github/workflows/php.yml
@@ -1,42 +1,49 @@
 name: PHP Composer
 
-on:
-  push:
-    branches: [ master ]
-  pull_request:
-    branches: [ master ]
+on: [push, pull_request]
 
 jobs:
   build:
     name: ViesBuild
     runs-on: ${{ matrix.operating-system }}
+    # Let the experimental (next-PHP) leg fail without failing the workflow.
+    # Legs without an "experimental" value resolve to false and stay blocking.
+    continue-on-error: ${{ matrix.experimental || false }}
     strategy:
+      fail-fast: false
       matrix:
         operating-system: [ubuntu-latest]
-        php-versions: ['7.4', '8.0', '8.1']
+        php: ['7.4', '8.0', '8.1', '8.2', '8.3', '8.4', '8.5']
+        include:
+          # Forward-looking, non-blocking signal against the next PHP.
+          # This plugin exists to smooth installs on newer PHP, so an
+          # early-warning leg against the upcoming release fits its purpose.
+          -   operating-system: ubuntu-latest
+              php: '8.6'
+              experimental: true
 
     steps:
-    - name: Set default PHP version ${{ matrix.php-versions }}
+    - name: Set default PHP version ${{ matrix.php }}
       uses: shivammathur/setup-php@v2
       with:
-        php-version: "${{ matrix.php-versions }}"
+        php-version: "${{ matrix.php }}"
 
     - name: Display current PHP version
       run: php --version
 
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v5
 
     - name: Validate composer.json and composer.lock
       run: composer validate
 
     - name: Cache Composer packages
       id: composer-cache
-      uses: actions/cache@v2
+      uses: actions/cache@v5
       with:
         path: vendor
-        key: ${{ runner.os }}-php-${{ hashFiles('**/composer.lock') }}
+        key: ${{ runner.os }}-php-${{ matrix.php }}-${{ hashFiles('**/composer.json') }}
         restore-keys: |
-          ${{ runner.os }}-php-
+          ${{ runner.os }}-php-${{ matrix.php }}-
 
     - name: Install dependencies
       if: steps.composer-cache.outputs.cache-hit != 'true'
diff --git a/src/Vies/Validator/ValidatorIE.php b/src/Vies/Validator/ValidatorIE.php
index 10eb54c..b1bec55 100644
--- a/src/Vies/Validator/ValidatorIE.php
+++ b/src/Vies/Validator/ValidatorIE.php
@@ -76,7 +76,9 @@ private function validateIENew(string $vatNumber): bool
 
         $checkChar = 'A';
         for ($i = $checkVal - 1; $i > 0; $i--) {
-            $checkChar++;
+            // chr(ord(...) + 1) replaces $checkChar++ to avoid incrementing a
+            // non-numeric string, deprecated since PHP 8.3 (valid on PHP 7.3+).
+            $checkChar = chr(ord($checkChar) + 1);
         }
 
         return $checkChar == $checksum;
diff --git a/src/Vies/Validator/ValidatorSK.php b/src/Vies/Validator/ValidatorSK.php
index da923dc..e490b44 100644
--- a/src/Vies/Validator/ValidatorSK.php
+++ b/src/Vies/Validator/ValidatorSK.php
@@ -35,6 +35,7 @@ class ValidatorSK extends ValidatorAbstract
     public function validate(string $vatNumber): bool
     {
         if (strlen($vatNumber) != 10
+            || ! ctype_digit($vatNumber)
             || intval($vatNumber[0]) == 0
             || ! in_array((int) $vatNumber[2], [2, 3, 4, 7, 8, 9])
         ) {
diff --git a/tests/Vies/Validator/ValidatorSKTest.php b/tests/Vies/Validator/ValidatorSKTest.php
index 26576e6..22c1b60 100644
--- a/tests/Vies/Validator/ValidatorSKTest.php
+++ b/tests/Vies/Validator/ValidatorSKTest.php
@@ -4,6 +4,8 @@
 
 namespace DragonBe\Test\Vies\Validator;
 
+use DragonBe\Vies\Validator\ValidatorSK;
+
 class ValidatorSKTest extends AbstractValidatorTest
 {
     /**
@@ -25,4 +27,25 @@ public function vatNumberProvider()
             ['4060000007', false],
         ];
     }
+
+    /**
+     * @covers \DragonBe\Vies\Validator\ValidatorSK
+     */
+    public function testValidateRaisesNoWarningOnNonNumericInput()
+    {
+        $raised = null;
+        set_error_handler(static function (int $errno, string $errstr) use (&$raised): bool {
+            $raised = $errstr;
+            return true;
+        }, E_DEPRECATED | E_WARNING);
+
+        try {
+            $result = (new ValidatorSK())->validate('222222222A');
+        } finally {
+            restore_error_handler();
+        }
+
+        self::assertFalse($result, 'Non-numeric SK input must be invalid');
+        self::assertNull($raised, 'Validator must not trigger E_WARNING or E_DEPRECATED');
+    }
 }
diff --git a/tests/Vies/ViesTest.php b/tests/Vies/ViesTest.php
index 47f2fd1..cfd3ac0 100644
--- a/tests/Vies/ViesTest.php
+++ b/tests/Vies/ViesTest.php
@@ -389,7 +389,9 @@ public function testCanAddOptionalArgumentsWithValue()
     {
         $viesRef = new \ReflectionClass(Vies::class);
         $addOptionalArguments = $viesRef->getMethod('addOptionalArguments');
-        $addOptionalArguments->setAccessible(true);
+        if (\PHP_VERSION_ID < 80100) {
+            $addOptionalArguments->setAccessible(true);
+        }
 
         $array = [];
         $object = new Vies();
@@ -409,7 +411,9 @@ public function testCanNotAddOptionalArgumentsWithoutValue()
     {
         $viesRef = new \ReflectionClass(Vies::class);
         $addOptionalArguments = $viesRef->getMethod('addOptionalArguments');
-        $addOptionalArguments->setAccessible(true);
+        if (\PHP_VERSION_ID < 80100) {
+            $addOptionalArguments->setAccessible(true);
+        }
 
         $array = [];
         $object = new Vies();
@@ -484,7 +488,9 @@ public function testRejectBadOptionalInformation(
     ) {
         $viesRef = new \ReflectionClass(Vies::class);
         $addOptionalArguments = $viesRef->getMethod('addOptionalArguments');
-        $addOptionalArguments->setAccessible(true);
+        if (\PHP_VERSION_ID < 80100) {
+            $addOptionalArguments->setAccessible(true);
+        }
 
         $array = [];
         $object = new Vies();
@@ -538,7 +544,9 @@ public function testAllowValidOptionalInformation(
     ) {
         $viesRef = new \ReflectionClass(Vies::class);
         $addOptionalArguments = $viesRef->getMethod('addOptionalArguments');
-        $addOptionalArguments->setAccessible(true);
+        if (\PHP_VERSION_ID < 80100) {
+            $addOptionalArguments->setAccessible(true);
+        }
 
         $array = [];
         $object = new Vies();
@@ -561,7 +569,9 @@ public function testBreakValidationOfOptionalArguments()
     {
         $viesRef = new \ReflectionClass(Vies::class);
         $addOptionalArguments = $viesRef->getMethod('addOptionalArguments');
-        $addOptionalArguments->setAccessible(true);
+        if (\PHP_VERSION_ID < 80100) {
+            $addOptionalArguments->setAccessible(true);
+        }
 
         $this->expectException(\TypeError::class);
         $array = [];