diff --git a/.github/FUNDING.yml b/.github/FUNDING.yml
deleted file mode 100644
index a8ff52a..0000000
--- a/.github/FUNDING.yml
+++ /dev/null
@@ -1 +0,0 @@
-github: casperbakker
diff --git a/src/Barcode.php b/src/Barcode.php
index dbb27f1..a39b382 100644
--- a/src/Barcode.php
+++ b/src/Barcode.php
@@ -14,7 +14,7 @@ public function __construct(string $barcode)
         $this->barcode = $barcode;
     }
 
-    public function addBar(BarcodeBar $bar)
+    public function addBar(BarcodeBar $bar): void
     {
         $this->bars[] = $bar;
         $this->width += $bar->getWidth();
diff --git a/src/BarcodeBar.php b/src/BarcodeBar.php
index 61a415f..58a881f 100644
--- a/src/BarcodeBar.php
+++ b/src/BarcodeBar.php
@@ -2,7 +2,7 @@
 
 namespace Picqer\Barcode;
 
-class BarcodeBar
+readonly class BarcodeBar
 {
     protected int $width;
     protected int $height;
diff --git a/src/BarcodeGenerator.php b/src/BarcodeGenerator.php
index 9dd7f76..8555b06 100644
--- a/src/BarcodeGenerator.php
+++ b/src/BarcodeGenerator.php
@@ -45,6 +45,7 @@
 use Picqer\Barcode\Types\TypeEan13;
 use Picqer\Barcode\Types\TypeEan8;
 use Picqer\Barcode\Types\TypeIntelligentMailBarcode;
+use Picqer\Barcode\Types\TypeInterface;
 use Picqer\Barcode\Types\TypeInterleaved25;
 use Picqer\Barcode\Types\TypeInterleaved25Checksum;
 use Picqer\Barcode\Types\TypeITF14;
@@ -101,6 +102,9 @@ abstract class BarcodeGenerator
     const TYPE_PHARMA_CODE = 'PHARMA';
     const TYPE_PHARMA_CODE_TWO_TRACKS = 'PHARMA2T';
 
+    /**
+     * @throws UnknownTypeException
+     */
     protected function getBarcodeData(string $code, string $type): Barcode
     {
         $barcodeDataBuilder = $this->createDataBuilderForType($type);
@@ -108,7 +112,7 @@ protected function getBarcodeData(string $code, string $type): Barcode
         return $barcodeDataBuilder->getBarcode($code);
     }
 
-    protected function createDataBuilderForType(string $type)
+    protected function createDataBuilderForType(string $type): TypeInterface
     {
         switch (strtoupper($type)) {
             case self::TYPE_CODE_32:
diff --git a/src/BarcodeGeneratorDynamicHTML.php b/src/BarcodeGeneratorDynamicHTML.php
index 66845a2..c08a4ed 100644
--- a/src/BarcodeGeneratorDynamicHTML.php
+++ b/src/BarcodeGeneratorDynamicHTML.php
@@ -2,6 +2,8 @@
 
 namespace Picqer\Barcode;
 
+use Picqer\Barcode\Exceptions\UnknownTypeException;
+
 class BarcodeGeneratorDynamicHTML extends BarcodeGenerator
 {
     /**
@@ -12,6 +14,7 @@ class BarcodeGeneratorDynamicHTML extends BarcodeGenerator
      * @param BarcodeGenerator::TYPE_* $type (string) type of barcode
      * @param string $foregroundColor Foreground color for bar elements as '#333' or 'orange' for example (background is transparent).
      * @return string HTML code.
+     * @throws UnknownTypeException
      */
     public function getBarcode(string $barcode, $type, string $foregroundColor = 'black'): string
     {
diff --git a/src/BarcodeGeneratorHTML.php b/src/BarcodeGeneratorHTML.php
index c38a723..bc30f25 100644
--- a/src/BarcodeGeneratorHTML.php
+++ b/src/BarcodeGeneratorHTML.php
@@ -2,6 +2,8 @@
 
 namespace Picqer\Barcode;
 
+use Picqer\Barcode\Exceptions\UnknownTypeException;
+
 class BarcodeGeneratorHTML extends BarcodeGenerator
 {
     /**
@@ -14,6 +16,7 @@ class BarcodeGeneratorHTML extends BarcodeGenerator
      * @param int $height Height of a single bar element in pixels.
      * @param string $foregroundColor Foreground color for bar elements as '#333' or 'orange' for example (background is transparent).
      * @return string HTML code.
+     * @throws UnknownTypeException
      */
     public function getBarcode(string $barcode, $type, int $widthFactor = 2, int $height = 30, string $foregroundColor = 'black'): string
     {
diff --git a/src/BarcodeGeneratorJPG.php b/src/BarcodeGeneratorJPG.php
index ba89b14..2e27001 100644
--- a/src/BarcodeGeneratorJPG.php
+++ b/src/BarcodeGeneratorJPG.php
@@ -2,6 +2,8 @@
 
 namespace Picqer\Barcode;
 
+use Picqer\Barcode\Exceptions\UnknownTypeException;
+
 class BarcodeGeneratorJPG extends BarcodeGenerator
 {
     protected ?bool $useImagick = null;
@@ -15,6 +17,7 @@ class BarcodeGeneratorJPG extends BarcodeGenerator
      * @param int $height Height of a single bar element in pixels.
      * @param array $foregroundColor RGB (0-255) foreground color for bar elements (background is transparent).
      * @return string image data or false in case of error.
+     * @throws UnknownTypeException
      */
     public function getBarcode(string $barcode, $type, int $widthFactor = 2, int $height = 30, array $foregroundColor = [0, 0, 0]): string
     {
@@ -37,7 +40,7 @@ public function getBarcode(string $barcode, $type, int $widthFactor = 2, int $he
     /**
      * Force the use of Imagick image extension
      */
-    public function useImagick()
+    public function useImagick(): void
     {
         $this->useImagick = true;
     }
@@ -45,7 +48,7 @@ public function useImagick()
     /**
      * Force the use of the GD image library
      */
-    public function useGd()
+    public function useGd(): void
     {
         $this->useImagick = false;
     }
diff --git a/src/BarcodeGeneratorPNG.php b/src/BarcodeGeneratorPNG.php
index f2c0b39..31edc56 100644
--- a/src/BarcodeGeneratorPNG.php
+++ b/src/BarcodeGeneratorPNG.php
@@ -2,6 +2,8 @@
 
 namespace Picqer\Barcode;
 
+use Picqer\Barcode\Exceptions\UnknownTypeException;
+
 class BarcodeGeneratorPNG extends BarcodeGenerator
 {
     protected ?bool $useImagick = null;
@@ -15,6 +17,7 @@ class BarcodeGeneratorPNG extends BarcodeGenerator
      * @param int $height Height of a single bar element in pixels.
      * @param array $foregroundColor RGB (0-255) foreground color for bar elements (background is transparent).
      * @return string image data or false in case of error.
+     * @throws UnknownTypeException
      */
     public function getBarcode(string $barcode, $type, int $widthFactor = 2, int $height = 30, array $foregroundColor = [0, 0, 0]): string
     {
@@ -37,7 +40,7 @@ public function getBarcode(string $barcode, $type, int $widthFactor = 2, int $he
     /**
      * Force the use of Imagick image extension
      */
-    public function useImagick()
+    public function useImagick(): void
     {
         $this->useImagick = true;
     }
@@ -45,7 +48,7 @@ public function useImagick()
     /**
      * Force the use of the GD image library
      */
-    public function useGd()
+    public function useGd(): void
     {
         $this->useImagick = false;
     }
diff --git a/src/BarcodeGeneratorSVG.php b/src/BarcodeGeneratorSVG.php
index aab6a15..ef7d63f 100644
--- a/src/BarcodeGeneratorSVG.php
+++ b/src/BarcodeGeneratorSVG.php
@@ -2,6 +2,8 @@
 
 namespace Picqer\Barcode;
 
+use Picqer\Barcode\Exceptions\UnknownTypeException;
+
 class BarcodeGeneratorSVG extends BarcodeGenerator
 {
     /**
@@ -14,6 +16,7 @@ class BarcodeGeneratorSVG extends BarcodeGenerator
      * @param $foregroundColor (string) Foreground color (in SVG format) for bar elements (background is transparent).
      * @return string SVG code.
      * @public
+     * @throws UnknownTypeException
      */
     public function getBarcode(string $barcode, $type, float $widthFactor = 2, float $height = 30, string $foregroundColor = 'black'): string
     {
diff --git a/src/Types/TypeCodabar.php b/src/Types/TypeCodabar.php
index 62c78f8..e782dc4 100644
--- a/src/Types/TypeCodabar.php
+++ b/src/Types/TypeCodabar.php
@@ -13,7 +13,7 @@
 
 class TypeCodabar implements TypeInterface
 {
-    protected $conversionTable = [
+    protected array $conversionTable = [
         '0' => '11111221',
         '1' => '11112211',
         '2' => '11121121',
diff --git a/src/Types/TypeCode11.php b/src/Types/TypeCode11.php
index 1886364..ab469bd 100644
--- a/src/Types/TypeCode11.php
+++ b/src/Types/TypeCode11.php
@@ -13,7 +13,7 @@
 
 class TypeCode11 implements TypeInterface
 {
-    protected $conversionTable = [
+    protected array $conversionTable = [
         '0' => '111121',
         '1' => '211121',
         '2' => '121121',
diff --git a/src/Types/TypeCode128.php b/src/Types/TypeCode128.php
index 3a6242d..ecc084e 100644
--- a/src/Types/TypeCode128.php
+++ b/src/Types/TypeCode128.php
@@ -18,9 +18,9 @@
 
 class TypeCode128 implements TypeInterface
 {
-    protected $type = null;
+    protected ?string $type = null;
 
-    protected $conversionTable = [
+    protected array $conversionTable = [
         '212222', /* 00 */
         '222122', /* 01 */
         '222221', /* 02 */
@@ -383,7 +383,7 @@ public function getBarcode(string $code): Barcode
      * @return array sequence
      * @protected
      */
-    protected function get128ABsequence($code)
+    protected function get128ABsequence($code): array
     {
         $len = strlen($code);
         $sequence = [];
diff --git a/src/Types/TypeCode128A.php b/src/Types/TypeCode128A.php
index ff2fe0c..78bb46e 100644
--- a/src/Types/TypeCode128A.php
+++ b/src/Types/TypeCode128A.php
@@ -12,5 +12,5 @@
 
 class TypeCode128A extends TypeCode128
 {
-    protected $type = 'A';
+    protected ?string $type = 'A';
 }
diff --git a/src/Types/TypeCode128B.php b/src/Types/TypeCode128B.php
index b462863..6dbc0d8 100644
--- a/src/Types/TypeCode128B.php
+++ b/src/Types/TypeCode128B.php
@@ -12,5 +12,5 @@
 
 class TypeCode128B extends TypeCode128
 {
-    protected $type = 'B';
+    protected ?string $type = 'B';
 }
diff --git a/src/Types/TypeCode128C.php b/src/Types/TypeCode128C.php
index f4d4460..82566a2 100644
--- a/src/Types/TypeCode128C.php
+++ b/src/Types/TypeCode128C.php
@@ -12,5 +12,5 @@
 
 class TypeCode128C extends TypeCode128
 {
-    protected $type = 'C';
+    protected ?string $type = 'C';
 }
diff --git a/src/Types/TypeCode32.php b/src/Types/TypeCode32.php
index 5c50395..54c9bde 100644
--- a/src/Types/TypeCode32.php
+++ b/src/Types/TypeCode32.php
@@ -13,7 +13,7 @@
  */
 class TypeCode32 extends TypeCode39
 {
-    protected $conversionTable32 = [
+    protected array $conversionTable32 = [
         '0' => '0',
         '1' => '1',
         '2' => '2',
diff --git a/src/Types/TypeCode39.php b/src/Types/TypeCode39.php
index 53cbaf9..2be9cc3 100644
--- a/src/Types/TypeCode39.php
+++ b/src/Types/TypeCode39.php
@@ -14,10 +14,10 @@
 
 class TypeCode39 implements TypeInterface
 {
-    protected $extended = false;
-    protected $checksum = false;
+    protected bool $extended = false;
+    protected bool $checksum = false;
 
-    protected $conversionTable = [
+    protected array $conversionTable = [
         '0' => '111331311',
         '1' => '311311113',
         '2' => '113311113',
@@ -113,10 +113,11 @@ public function getBarcode(string $code): Barcode
      * Encode a string to be used for CODE 39 Extended mode.
      *
      * @param string $code code to represent.
-     * @return bool|string encoded string.
+     * @return string encoded string.
      * @protected
+     * @throws InvalidCharacterException
      */
-    protected function encode_code39_ext($code)
+    protected function encode_code39_ext(string $code): string
     {
         $encode = [
             chr(0) => '%U',
diff --git a/src/Types/TypeCode39Checksum.php b/src/Types/TypeCode39Checksum.php
index b129f28..7a47bb6 100644
--- a/src/Types/TypeCode39Checksum.php
+++ b/src/Types/TypeCode39Checksum.php
@@ -9,6 +9,6 @@
 
 class TypeCode39Checksum extends TypeCode39
 {
-    protected $extended = false;
-    protected $checksum = true;
+    protected bool $extended = false;
+    protected bool $checksum = true;
 }
diff --git a/src/Types/TypeCode39Extended.php b/src/Types/TypeCode39Extended.php
index d16cfd6..0d85043 100644
--- a/src/Types/TypeCode39Extended.php
+++ b/src/Types/TypeCode39Extended.php
@@ -9,6 +9,6 @@
 
 class TypeCode39Extended extends TypeCode39
 {
-    protected $extended = true;
-    protected $checksum = false;
+    protected bool $extended = true;
+    protected bool $checksum = false;
 }
diff --git a/src/Types/TypeCode39ExtendedChecksum.php b/src/Types/TypeCode39ExtendedChecksum.php
index 1d28a88..9e5292c 100644
--- a/src/Types/TypeCode39ExtendedChecksum.php
+++ b/src/Types/TypeCode39ExtendedChecksum.php
@@ -9,6 +9,6 @@
 
 class TypeCode39ExtendedChecksum extends TypeCode39
 {
-    protected $extended = true;
-    protected $checksum = true;
+    protected bool $extended = true;
+    protected bool $checksum = true;
 }
diff --git a/src/Types/TypeCode93.php b/src/Types/TypeCode93.php
index 0bd9317..f944cf7 100644
--- a/src/Types/TypeCode93.php
+++ b/src/Types/TypeCode93.php
@@ -15,7 +15,7 @@
 
 class TypeCode93 implements TypeInterface
 {
-    protected $conversionTable = [
+    protected array $conversionTable = [
         48 => '131112', // 0
         49 => '111213', // 1
         50 => '111312', // 2
@@ -246,7 +246,7 @@ public function getBarcode(string $code): Barcode
      * @return string checksum code.
      * @protected
      */
-    protected function checksum_code93($code)
+    protected function checksum_code93(string $code): string
     {
         $chars = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', '-', '.', ' ', '$', '/', '+', '%', 'a', 'b', 'c', 'd'];
 
@@ -280,8 +280,6 @@ protected function checksum_code93($code)
         $check %= 47;
         $k = $chars[$check];
 
-        $checksum = $c . $k;
-
-        return $checksum;
+        return $c . $k;
     }
 }
diff --git a/src/Types/TypeEanUpcBase.php b/src/Types/TypeEanUpcBase.php
index fe8948a..fba0121 100644
--- a/src/Types/TypeEanUpcBase.php
+++ b/src/Types/TypeEanUpcBase.php
@@ -211,7 +211,7 @@ protected function calculateChecksumDigit(string $code): int
         // calculate check digit
         $sum_a = 0;
         for ($i = 1; $i < $this->length - 1; $i += 2) {
-            $sum_a += $code[$i];
+            $sum_a += intval($code[$i]);
         }
         if ($this->length > 12) {
             $sum_a *= 3;
diff --git a/src/Types/TypeInterleaved25Checksum.php b/src/Types/TypeInterleaved25Checksum.php
index 5871e9b..10727e6 100644
--- a/src/Types/TypeInterleaved25Checksum.php
+++ b/src/Types/TypeInterleaved25Checksum.php
@@ -75,11 +75,11 @@ protected function getChecksum(string $code): string
         $len = strlen($code);
         $sum = 0;
         for ($i = 0; $i < $len; $i += 2) {
-            $sum += $code[$i];
+            $sum += intval($code[$i]);
         }
         $sum *= 3;
         for ($i = 1; $i < $len; $i += 2) {
-            $sum += ($code[$i]);
+            $sum += intval($code[$i]);
         }
         $r = $sum % 10;
         if ($r > 0) {
diff --git a/src/Types/TypeMsi.php b/src/Types/TypeMsi.php
index d6e91cb..12f6ee6 100644
--- a/src/Types/TypeMsi.php
+++ b/src/Types/TypeMsi.php
@@ -13,5 +13,5 @@
 
 class TypeMsi extends TypeMsiChecksum
 {
-    protected $checksum = false;
+    protected bool $checksum = false;
 }
diff --git a/src/Types/TypeMsiChecksum.php b/src/Types/TypeMsiChecksum.php
index c421391..3d06f07 100644
--- a/src/Types/TypeMsiChecksum.php
+++ b/src/Types/TypeMsiChecksum.php
@@ -17,7 +17,7 @@
 
 class TypeMsiChecksum implements TypeInterface
 {
-    protected $checksum = true;
+    protected bool $checksum = true;
 
     public function getBarcode(string $code): Barcode
     {
diff --git a/src/Types/TypePlanet.php b/src/Types/TypePlanet.php
index 0752386..c3b7f5e 100644
--- a/src/Types/TypePlanet.php
+++ b/src/Types/TypePlanet.php
@@ -13,7 +13,7 @@
 
 class TypePlanet extends TypePostnet
 {
-    protected $barlen = [
+    protected array $barlen = [
         0 => [1, 1, 2, 2, 2],
         1 => [2, 2, 2, 1, 1],
         2 => [2, 2, 1, 2, 1],
diff --git a/src/Types/TypePostnet.php b/src/Types/TypePostnet.php
index 2d136b3..0d235d8 100644
--- a/src/Types/TypePostnet.php
+++ b/src/Types/TypePostnet.php
@@ -16,7 +16,7 @@
 
 class TypePostnet implements TypeInterface
 {
-    protected $barlen = [
+    protected array $barlen = [
         0 => [2, 2, 1, 1, 1],
         1 => [1, 1, 1, 2, 2],
         2 => [1, 1, 2, 1, 2],
diff --git a/src/Types/TypeStandard2of5.php b/src/Types/TypeStandard2of5.php
index 906445d..1445a3c 100644
--- a/src/Types/TypeStandard2of5.php
+++ b/src/Types/TypeStandard2of5.php
@@ -14,7 +14,7 @@
 
 class TypeStandard2of5 implements TypeInterface
 {
-    protected $checksum = false;
+    protected bool $checksum = false;
 
     public function getBarcode(string $code): Barcode
     {
@@ -50,25 +50,25 @@ public function getBarcode(string $code): Barcode
      * Checksum for standard 2 of 5 barcodes.
      *
      * @param $code (string) code to process.
-     * @return int checksum.
+     * @return string checksum.
      * @protected
      */
-    protected function checksum_s25($code)
+    protected function checksum_s25(string $code): string
     {
         $len = strlen($code);
         $sum = 0;
         for ($i = 0; $i < $len; $i += 2) {
-            $sum += $code[$i];
+            $sum += intval($code[$i]);
         }
         $sum *= 3;
         for ($i = 1; $i < $len; $i += 2) {
-            $sum += ($code[$i]);
+            $sum += intval($code[$i]);
         }
         $r = $sum % 10;
         if ($r > 0) {
             $r = (10 - $r);
         }
 
-        return $r;
+        return strval($r);
     }
 }
diff --git a/src/Types/TypeStandard2of5Checksum.php b/src/Types/TypeStandard2of5Checksum.php
index 25bc4a0..c1b6fc5 100644
--- a/src/Types/TypeStandard2of5Checksum.php
+++ b/src/Types/TypeStandard2of5Checksum.php
@@ -10,5 +10,5 @@
 
 class TypeStandard2of5Checksum extends TypeStandard2of5
 {
-    protected $checksum = true;
+    protected bool $checksum = true;
 }
diff --git a/src/Types/TypeTelepen.php b/src/Types/TypeTelepen.php
index 378e342..477d4b9 100644
--- a/src/Types/TypeTelepen.php
+++ b/src/Types/TypeTelepen.php
@@ -23,11 +23,12 @@ class TypeTelepen implements TypeInterface
 {
     private const TELEPEN_START_CHAR = '_';
     private const TELEPEN_STOP_CHAR = 'z';
+
     private const TELEPEN_ALPHA = 'alpha';
     private const TELEPEN_NUMERIC = 'numeric';
 
-    private $telepen_lookup_table;
-    private $mode;
+    private array $telepen_lookup_table;
+    private string $mode;
 
     public function __construct($m = 'alpha')
     {
@@ -59,7 +60,6 @@ public function getBarcode(string $code): Barcode
 
     protected function encode($code) : string
     {
-        $result = null;
         if ($this->mode == self::TELEPEN_ALPHA) {
             $result = $this->encodeAlpha($code);
         } else {
@@ -160,7 +160,7 @@ private function encodeNumeric(string $code) : string
      * More information about Telepen symbology is available from
      * https://v4l237.n3cdn1.secureserver.net/wp-content/uploads/2022/05/Barcode-Symbology-information-and-History.pdf
      */
-    private function createTelepenConversionTable()
+    private function createTelepenConversionTable(): void
     {
         $this->telepen_lookup_table = [
             "1111111111111111", "1131313111", "33313111", "1111313131", 
diff --git a/src/Types/TypeUpcExtension2.php b/src/Types/TypeUpcExtension2.php
index f0620e1..22ecbca 100644
--- a/src/Types/TypeUpcExtension2.php
+++ b/src/Types/TypeUpcExtension2.php
@@ -14,7 +14,7 @@
 
 class TypeUpcExtension2 implements TypeInterface
 {
-    protected $length = 2;
+    protected int $length = 2;
 
     public function getBarcode(string $code): Barcode
     {
diff --git a/src/Types/TypeUpcExtension5.php b/src/Types/TypeUpcExtension5.php
index b1d3424..c9e3065 100644
--- a/src/Types/TypeUpcExtension5.php
+++ b/src/Types/TypeUpcExtension5.php
@@ -10,5 +10,5 @@
 
 class TypeUpcExtension5 extends TypeUpcExtension2
 {
-    protected $length = 5;
+    protected int $length = 5;
 }