diff --git a/src/PublicSuffixList.php b/src/PublicSuffixList.php
index 360ecbf..00afdf3 100644
--- a/src/PublicSuffixList.php
+++ b/src/PublicSuffixList.php
@@ -1,4 +1,5 @@
-<?php
+<?php declare(strict_types=1);
+
 namespace Xoops\RegDom;
 
 /**
@@ -13,19 +14,17 @@
  */
 class PublicSuffixList
 {
-    protected $sourceURL = 'https://publicsuffix.org/list/public_suffix_list.dat';
-    protected $localPSL = 'public_suffix_list.dat';
-    protected $cachedPrefix = 'cached_';
-
-    protected $tree;
-    protected $url;
-    protected $dataDir = '/../data/'; // relative to __DIR__
-
+    private string $sourceURL = 'https://publicsuffix.org/list/public_suffix_list.dat';
+    private string $localPSL = 'public_suffix_list.dat';
+    private string $cachedPrefix = 'cached_';
+    private ?array $tree = null;
+    private ?string $url = null;
+    private string $dataDir = '/../data/'; // relative to __DIR__
     /**
      * PublicSuffixList constructor.
      * @param string|null $url URL for the PSL or null to use default
      */
-    public function __construct($url = null)
+    public function __construct(?string $url = null)
     {
         $this->setURL($url);
     }
@@ -34,10 +33,9 @@ public function __construct($url = null)
      * Set the URL, and clear any existing tree
      *
      * @param string|null $url URL for the PSL or null to use default
-     *
      * @return void
      */
-    public function setURL($url)
+    public function setURL(?string $url): void
     {
         $this->url = $url;
         $this->tree = null;
@@ -49,7 +47,7 @@ public function setURL($url)
      *
      * @return void
      */
-    protected function setFallbackURL()
+    private function setFallbackURL(): void
     {
         $this->setLocalPSLName($this->url);
         if (null === $this->url) {
@@ -61,15 +59,14 @@ protected function setFallbackURL()
      * Load the PSL tree, automatically handling caches
      *
      * @return void (results in $this->tree)
-     *
      * @throws \RuntimeException
      */
-    protected function loadTree()
+    private function loadTree(): void
     {
         $this->setFallbackURL();
 
         $this->tree = $this->readCachedPSL($this->url);
-        if (false !== $this->tree) {
+        if (null !== $this->tree) {
             return;
         }
 
@@ -88,15 +85,14 @@ protected function loadTree()
      * Parse the PSL data
      *
      * @param string $fileData the PSL data
-     *
      * @return void (results in $this->tree)
      */
-    protected function parsePSL($fileData)
+    private function parsePSL(string $fileData): void
     {
         $lines = explode("\n", $fileData);
 
         foreach ($lines as $line) {
-            if ($this->startsWith($line, '//') || '' == $line) {
+            if ('' === $line || $this->startsWith($line, '//'))   {
                 continue;
             }
 
@@ -110,12 +106,11 @@ protected function parsePSL($fileData)
     /**
      * Does $search start with $startString?
      *
-     * @param string $search      the string to test
+     * @param string $search the string to test
      * @param string $startString the starting string to match
-     *
      * @return bool
      */
-    protected function startsWith($search, $startString)
+    private function startsWith(string $search, string $startString): bool
     {
         return (0 === strpos($search, $startString));
     }
@@ -123,12 +118,11 @@ protected function startsWith($search, $startString)
     /**
      * Add domains to tree
      *
-     * @param array    $node     tree array by reference
+     * @param array $node tree array by reference
      * @param string[] $tldParts array of domain parts
-     *
      * @return void - changes made to $node by reference
      */
-    protected function buildSubDomain(&$node, $tldParts)
+    private function buildSubDomain(array &$node, array $tldParts): void
     {
         $dom = trim(array_pop($tldParts));
 
@@ -146,7 +140,7 @@ protected function buildSubDomain(&$node, $tldParts)
             }
         }
 
-        if (!$isNotDomain && count($tldParts) > 0) {
+        if (!$isNotDomain && 0 < count($tldParts)) {
             $this->buildSubDomain($node[$dom], $tldParts);
         }
     }
@@ -157,7 +151,7 @@ protected function buildSubDomain(&$node, $tldParts)
      * @return array the PSL tree
      * @throws \RuntimeException if PSL cannot be loaded
      */
-    public function getTree()
+    public function getTree(): array
     {
         if (null === $this->tree) {
             $this->loadTree();
@@ -171,7 +165,7 @@ public function getTree()
      *
      * @return string|false PSL file contents or false on error
      */
-    protected function readPSL()
+    private function readPSL()
     {
         $parts = parse_url($this->url);
         $remote = isset($parts['scheme']) || isset($parts['host']);
@@ -206,10 +200,9 @@ protected function readPSL()
      * Determine cache file name for a specified source
      *
      * @param string $url URL/filename of source PSL
-     *
      * @return string cache file name for given resource
      */
-    protected function getCacheFileName($url)
+    private function getCacheFileName(string $url): string
     {
         return __DIR__ . $this->dataDir . $this->cachedPrefix . md5($url);
     }
@@ -218,26 +211,23 @@ protected function getCacheFileName($url)
      * Attempt to load a cached Public Suffix List tree for a given source
      *
      * @param string $url URL/filename of source PSL
-     *
      * @return false|string[] PSL tree
      */
-    protected function readCachedPSL($url)
+    private function readCachedPSL(string $url)
     {
         $cacheFile = $this->getCacheFileName($url);
         return file_exists($cacheFile)
             ? unserialize(file_get_contents($cacheFile), ['allowed_classes' => false])
-            : false;
+            : null;
     }
 
-
     /**
      * Cache the current Public Suffix List tree and associate with the specified source
      *
      * @param string $url URL/filename of source PSL
-     *
      * @return bool|int the number of bytes that were written to the file, or false on failure
      */
-    protected function cachePSL($url)
+    private function cachePSL(string $url)
     {
         return file_put_contents($this->getCacheFileName($url), serialize($this->tree));
     }
@@ -246,10 +236,9 @@ protected function cachePSL($url)
      * Save a local copy of a retrieved Public Suffix List
      *
      * @param string $fileContents URL/filename of source PSL
-     *
      * @return bool|int the number of bytes that were written to the file, or false on failure
      */
-    protected function saveLocalPSL($fileContents)
+    private function saveLocalPSL(string $fileContents)
     {
         return file_put_contents(__DIR__ . $this->localPSL, $fileContents);
     }
@@ -257,11 +246,10 @@ protected function saveLocalPSL($fileContents)
     /**
      * Set localPSL name based on URL
      *
-     * @param null|string $url the URL for the PSL
-     *
+     * @param string|null $url the URL for the PSL
      * @return void (sets $this->localPSL)
      */
-    protected function setLocalPSLName($url)
+    private function setLocalPSLName(?string $url): void
     {
         if (null === $url) {
             $url = $this->sourceURL;
@@ -275,17 +263,16 @@ protected function setLocalPSLName($url)
      * Delete files in the data directory
      *
      * @param bool $cacheOnly true to limit clearing to cached serialized PSLs, false to clear all
-     *
      * @return void
      */
-    public function clearDataDirectory($cacheOnly = false)
+    public function clearDataDirectory(bool $cacheOnly = false): void
     {
         $dir = __DIR__ . $this->dataDir;
         if (is_dir($dir)) {
-            if ($dirHandle = opendir($dir)) {
+            if (false !== ($dirHandle = opendir($dir))) {
                 while (false !== ($file = readdir($dirHandle))) {
                     if ('file' === filetype($dir . $file)
-                        && (false === $cacheOnly || $this->startsWith($file, $this->cachedPrefix))) {
+                        && (!$cacheOnly || $this->startsWith($file, $this->cachedPrefix))) {
                         unlink($dir . $file);
                     }
                 }
diff --git a/src/RegisteredDomain.php b/src/RegisteredDomain.php
index ab6dd2e..a405ece 100644
--- a/src/RegisteredDomain.php
+++ b/src/RegisteredDomain.php
@@ -1,4 +1,5 @@
-<?php
+<?php declare(strict_types=1);
+
 namespace Xoops\RegDom;
 
 /**
@@ -12,22 +13,20 @@
  * @author    Richard Griffith <richard@geekwright.com>
  * @license   Apache License, Version 2.0 (http://www.apache.org/licenses/LICENSE-2.0)
  */
+
 class RegisteredDomain
 {
-    protected $tree;
-    protected $psl;
+    private ?array $tree = null;
+    private PublicSuffixList $psl;
 
     /**
      * RegisteredDomain constructor.
      *
      * @param PublicSuffixList|null $psl PublicSuffixList object, or null to use defaults
      */
-    public function __construct(PublicSuffixList $psl = null)
+    public function __construct(?PublicSuffixList $psl = null)
     {
-        if (null === $psl) {
-            $psl = new PublicSuffixList();
-        }
-        $this->psl = $psl;
+        $this->psl = $psl ?? new PublicSuffixList();
     }
 
     /**
@@ -38,7 +37,7 @@ public function __construct(PublicSuffixList $psl = null)
      *
      * @return string
      */
-    protected function normalizeHost($url)
+    protected function normalizeHost(string $url): string
     {
         if (empty($url)) {
             return '';
@@ -59,20 +58,17 @@ protected function normalizeHost($url)
     }
 
     /**
-     * Convert a punycode string to UTF-8 if needed
+     * Convert a punycode string to UTF-8 if needed.
      *
-     * @param string $part host component
+     * @param string $part Host component
      *
-     * @return string host component as UTF-8
+     * @return string Host component as UTF-8
      */
-    protected function convertPunycode($part)
+    protected function convertPunycode(string $part): string
     {
         if (0 === strpos($part, 'xn--')) {
             if (function_exists('idn_to_utf8')) {
-                if (defined('INTL_IDNA_VARIANT_UTS46')) { // PHP 7.2
-                    return idn_to_utf8($part, 0, INTL_IDNA_VARIANT_UTS46);
-                }
-                return idn_to_utf8($part);
+                return defined('INTL_IDNA_VARIANT_UTS46') ? idn_to_utf8($part, 0, INTL_IDNA_VARIANT_UTS46) : idn_to_utf8($part);
             }
             return $this->decodePunycode($part);
         }
@@ -80,14 +76,14 @@ protected function convertPunycode($part)
     }
 
     /**
-     * convert punycode to UTF-8 (the hard way) Used only if idn_to_utf8() is not available
+     * Convert punycode to UTF-8 (the hard way) if idn_to_utf8() is not available.
      *
      * This fallback adapted from https://ckon.wordpress.com/2010/08/24/punycode-to-unicode-converter-php/
      *
      * @param string $encoded
      * @return string
      */
-    protected function decodePunycode($encoded)
+    protected function decodePunycode(string $encoded): string
     {
         $prefix = 'xn--';
         $safe_char = 0xFFFC;
@@ -97,7 +93,12 @@ protected function decodePunycode($encoded)
         $skew = 38;
         $damp = 700;
 
-        if (null === $encoded || 0 !== strpos($encoded, $prefix) || 0 == strlen(trim(str_replace($prefix, '', $encoded)))) {
+        if (0 !== strpos($encoded, $prefix)) {
+            return $encoded;
+        }
+
+        $trimmed = trim(str_replace($prefix, '', $encoded));
+        if ('' === $trimmed) {
             return $encoded;
         }
 
@@ -171,13 +172,13 @@ protected function decodePunycode($encoded)
     }
 
     /**
-     * Determine the registered domain portion of the supplied host string
+     * Determine the registered domain portion of the supplied host string.
      *
-     * @param string $host a host name or URL containing a host name
+     * @param string $host A host name or URL containing a host name
      *
-     * @return string|null shortest registrable domain portion of the supplied host or null if invalid
+     * @return string|null Shortest registrable domain portion of the supplied host or null if invalid
      */
-    public function getRegisteredDomain($host)
+    public function getRegisteredDomain(string $host): ?string
     {
         $this->tree = $this->psl->getTree();
 
@@ -194,7 +195,7 @@ public function getRegisteredDomain($host)
         // assure there is at least 1 TLD in the stripped signing domain
         if (!strpos($result, '.')) {
             $cnt = count($signingDomainParts);
-            if (1 == $cnt || '' == $signingDomainParts[$cnt-2]) {
+            if (1 === $cnt || '' === $signingDomainParts[$cnt-2]) {
                 return null;
             }
             return $signingDomainParts[$cnt-2] . '.' . $signingDomainParts[$cnt-1];
@@ -203,23 +204,23 @@ public function getRegisteredDomain($host)
     }
 
     /**
-     * Recursive helper method to query the PSL tree
+     * Recursive helper method to query the PSL tree.
      *
-     * @param string[] $remainingSigningDomainParts parts of domain being queried
-     * @param string[] $treeNode                    subset of tree array by reference
+     * @param string[] $remainingSigningDomainParts Parts of domain being queried
+     * @param array $treeNode Subset of tree array by reference
      *
-     * @return null|string
+     * @return string|null
      */
-    protected function findRegisteredDomain($remainingSigningDomainParts, &$treeNode)
+    protected function findRegisteredDomain(array $remainingSigningDomainParts, array &$treeNode): ?string
     {
         $sub = array_pop($remainingSigningDomainParts);
 
         $result = null;
         if (isset($treeNode['!'])) {
             return '';
-        } elseif (is_array($treeNode) && array_key_exists($sub, $treeNode)) {
+        } elseif (array_key_exists($sub, $treeNode)) {
             $result = $this->findRegisteredDomain($remainingSigningDomainParts, $treeNode[$sub]);
-        } elseif (is_array($treeNode) && array_key_exists('*', $treeNode)) {
+        } elseif (array_key_exists('*', $treeNode)) {
             $result = $this->findRegisteredDomain($remainingSigningDomainParts, $treeNode['*']);
         } else {
             return $sub;
diff --git a/tests/unit/PublicSuffixListTest.php b/tests/unit/PublicSuffixListTest.php
index 37eec15..88fbd4a 100644
--- a/tests/unit/PublicSuffixListTest.php
+++ b/tests/unit/PublicSuffixListTest.php
@@ -1,4 +1,5 @@
-<?php
+<?php declare(strict_types=1);
+
 namespace Xoops\RegDom;
 
 use PHPUnit\Framework\TestCase;
@@ -8,7 +9,7 @@ class PublicSuffixListTest extends TestCase
     /**
      * @var PublicSuffixList
      */
-    protected $object;
+    protected PublicSuffixList $object;
 
     /**
      * Sets up the fixture, for example, opens a network connection.
@@ -25,6 +26,7 @@ protected function setUp(): void
      */
     protected function tearDown(): void
     {
+//        $this->object->clearDataDirectory();
     }
 
     public function testContracts()
@@ -54,4 +56,39 @@ public function testClearDataDirectoryCacheOnly()
         $this->assertIsArray($tree);
         $this->assertArrayHasKey('com', $tree);
     }
+
+    public function testSetURL()
+    {
+        $url = 'https://example.com';
+        $this->object->setURL($url);
+
+        // Use reflection to call the protected setFallbackURL method
+        $reflection = new \ReflectionClass($this->object);
+        // Check the URL property to verify the fallback URL is set correctly
+        $property = $reflection->getProperty('url');
+        $property->setAccessible(true);
+
+        $this->assertSame($url, $property->getValue($this->object));
+
+    }
+
+    public function testFallbackURL()
+    {
+        // Set URL to null and trigger the fallback mechanism
+        $this->object->setURL(null);
+
+        // Use reflection to call the protected setFallbackURL method
+        $reflection = new \ReflectionClass($this->object);
+        $method = $reflection->getMethod('setFallbackURL');
+        $method->setAccessible(true);
+        $method->invoke($this->object);
+
+        // Check the URL property to verify the fallback URL is set correctly
+        $property = $reflection->getProperty('url');
+        $property->setAccessible(true);
+
+//        $expectedUrl = file_exists(dirname(__DIR__, 2) . '/data/public_suffix_list.dat') ? dirname(__DIR__, 2) . '/data/public_suffix_list.dat' : 'https://publicsuffix.org/list/public_suffix_list.dat';
+        $expectedUrl = file_exists(dirname(__DIR__, 2) . '/data/public_suffix_list.dat') ?  '/../data/public_suffix_list.dat' : 'https://publicsuffix.org/list/public_suffix_list.dat';
+        $this->assertSame($expectedUrl, $property->getValue($this->object));
+    }
 }
diff --git a/tests/unit/RegisteredDomainTest.php b/tests/unit/RegisteredDomainTest.php
index acd1057..e207f05 100644
--- a/tests/unit/RegisteredDomainTest.php
+++ b/tests/unit/RegisteredDomainTest.php
@@ -1,4 +1,5 @@
-<?php
+<?php declare(strict_types=1);
+
 namespace Xoops\RegDom;
 
 use Xoops\RegDom\RegisteredDomain;
@@ -8,24 +9,24 @@
 include __DIR__ . '/../../src/RegisteredDomain.php';
 include __DIR__ . '/../../src/PublicSuffixList.php';
 
+
 /**
  * Class TestProtectedDecodePunycode used to test protected decodePunycode() method that is
  * only used if intl extension is not loaded.
  */
 class TestProtectedDecodePunycode extends RegisteredDomain
 {
-    public function doDecodePunycode($string)
+    public function doDecodePunycode(string $string)
     {
         return $this->decodePunycode($string);
     }
 }
-
 class RegisteredDomainTest extends TestCase
 {
     /**
      * @var RegisteredDomain
      */
-    protected $object;
+    protected RegisteredDomain $object;
 
     /**
      * Sets up the fixture, for example, opens a network connection.
@@ -33,7 +34,7 @@ class RegisteredDomainTest extends TestCase
      */
     protected function setUp(): void
     {
-        //$this->object = new RegisteredDomain();
+        $this->object = new RegisteredDomain(new PublicSuffixList());
     }
 
     /**
@@ -42,12 +43,12 @@ protected function setUp(): void
      */
     protected function tearDown(): void
     {
+        // Clean up if necessary
     }
 
-    public function testContracts()
+    public function testContracts(): void
     {
-        $object = new RegisteredDomain();
-        $this->assertInstanceOf(RegisteredDomain::class, $object);
+        $this->assertInstanceOf(RegisteredDomain::class, $this->object);
     }
 
 
@@ -55,19 +56,18 @@ public function testContracts()
     /**
      * @dataProvider domainsProvider
      */
-    public function testGetRegisteredDomain($url = '', $regdom = '')
+    public function testGetRegisteredDomain(string $url, ?string $regdom): void
     {
-        $object = new RegisteredDomain();
-        $this->assertEquals($regdom, $object->getRegisteredDomain($url));
+        $this->assertEquals($regdom, $this->object->getRegisteredDomain($url));
     }
 
     /**
-     * @return array
+     * @return array<int, array{0: ?string, 1: ?string}>
      */
-    public static function domainsProvider()
+    public static function domainsProvider(): array
     {
-        $provider = [
-            [null, null],
+        return [
+            ['', ''],
             // Mixed case.
             ['COM', null],
             ['example.COM', 'example.com'],
@@ -158,26 +158,25 @@ public static function domainsProvider()
             ['rfu.in.ua', 'rfu.in.ua'],
             ['in.ua', null],
         ];
-        return $provider;
     }
 
 //    #[\PHPUnit\Framework\Attributes\DataProvider('punycodeProvider')] //PHP 8
     /**
      * @dataProvider punycodeProvider
      */
-    public function testDecodePunycode($punycode, $decoded)
+    public function testDecodePunycode(string $punycode, string $decoded): void
     {
-        $object = new TestProtectedDecodePunycode();
+        $object = new TestProtectedDecodePunycode(new PublicSuffixList());
         $this->assertEquals($decoded, $object->doDecodePunycode($punycode));
     }
 
     /**
-     * @return array
+     * @return array<int, array{0: ?string, 1: ?string}>
      */
-    public static function punycodeProvider()
+    public static function punycodeProvider(): array
     {
-        $provider = [
-            [null, null],
+        return [
+            ['', ''],
             // Mixed case.
             ['test', 'test'],
             // punycoded
@@ -185,6 +184,5 @@ public static function punycodeProvider()
             ['xn--55qx5d', '公司'],
             ['xn--fiqs8s', '中国'],
         ];
-        return $provider;
-    }
+}
 }