diff --git a/.gitignore b/.gitignore
index 4752cca..7b2cbac 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 .idea/
 /vendor
 /build
-composer.lock
\ No newline at end of file
+composer.lock
+*.cache
\ No newline at end of file
diff --git a/src/NumeroALetras.php b/src/NumeroALetras.php
index 9cf4371..1a67dab 100644
--- a/src/NumeroALetras.php
+++ b/src/NumeroALetras.php
@@ -6,8 +6,16 @@
 
 class NumeroALetras
 {
+
     /**
-     * @var array
+     * @var array $unidades
+     * 
+     * Array that contains the Spanish words for numbers from 0 to 20.
+     * Each index corresponds to the number it represents.
+     * 
+     * Example:
+     * - $unidades[1] returns 'UNO '
+     * - $unidades[10] returns 'DIEZ '
      */
     private $unidades = [
         '',
@@ -34,7 +42,11 @@ class NumeroALetras
     ];
 
     /**
-     * @var array
+     * @var array $decenas Array containing Spanish words for tens (decades).
+     * 
+     * This array is used to convert numbers into their corresponding Spanish words
+     * for tens. The values represent the words for twenty, thirty, forty, fifty,
+     * sixty, seventy, eighty, ninety, and one hundred.
      */
     private $decenas = [
         'VEINTI',
@@ -49,7 +61,9 @@ class NumeroALetras
     ];
 
     /**
-     * @var array
+     * Array of strings representing the hundreds in Spanish.
+     *
+     * @var string[]
      */
     private $centenas = [
         'CIENTO ',
@@ -64,6 +78,12 @@ class NumeroALetras
     ];
 
     /**
+     * Array of exceptions for accented words.
+     * 
+     * This array contains specific words that require accents in their 
+     * representation. The key is the word without the accent, and the 
+     * value is the word with the correct accent.
+     * 
      * @var array
      */
     private $acentosExcepciones = [
@@ -73,24 +93,27 @@ class NumeroALetras
     ];
 
     /**
-     * @var string
+     * @var string $conector The connector string used in the NumeroALetras class.
      */
     public $conector = 'CON';
 
     /**
-     * @var bool
+     * @var bool $apocope
+     * 
+     * This property determines whether the apocope (shortened form) of numbers should be used.
+     * When set to true, the apocope form will be applied.
+     * Default value is false.
      */
     public $apocope = false;
 
     /**
-     * Formatea y convierte un número a letras.
-     *
-     * @param int|float $number
-     * @param int       $decimals
+     * Converts a numeric value to its word representation.
      *
-     * @return string
+     * @param float|int $number The number to be converted.
+     * @param int $decimals The number of decimal places to consider. Default is 2.
+     * @return string The word representation of the number.
      */
-    public function toWords($number, $decimals = 2)
+    public function toWords(float|int $number, int $decimals = 2): string
     {
         $this->checkApocope();
 
@@ -104,20 +127,19 @@ public function toWords($number, $decimals = 2)
             $splitNumber[1] = $this->convertNumber($splitNumber[1]);
         }
 
-        return $this->glue($splitNumber);
+        return $this->concat($splitNumber);
     }
 
     /**
-     * Formatea y convierte un número a letras en formato moneda.
+     * Converts a numeric value to its money representation in words.
      *
-     * @param int|float $number
-     * @param int       $decimals
-     * @param string    $currency
-     * @param string    $cents
-     *
-     * @return string
+     * @param float $number The numeric value to be converted.
+     * @param int $decimals The number of decimal places to consider. Default is 2.
+     * @param string $currency The currency to append to the whole number part. Default is an empty string.
+     * @param string $cents The currency to append to the decimal part. Default is an empty string.
+     * @return string The money representation of the number in words.
      */
-    public function toMoney($number, $decimals = 2, $currency = '', $cents = '')
+    public function toMoney(float $number, int $decimals = 2, string $currency = '', string $cents = ''): string
     {
         $this->checkApocope();
 
@@ -135,34 +157,32 @@ public function toMoney($number, $decimals = 2, $currency = '', $cents = '')
             $splitNumber[1] .= ' ' . mb_strtoupper($cents, 'UTF-8');
         }
 
-        return $this->glue($splitNumber);
+        return $this->concat($splitNumber);
     }
 
     /**
-     * Formatea y convierte un número a letras en formato libre.
-     *
-     * @param int|float $number
-     * @param int       $decimals
-     * @param string    $whole_str
-     * @param string    $decimal_str
+     * Converts a number to its string representation.
      *
-     * @return string
+     * @param float|int $number The number to be converted.
+     * @param int $decimals The number of decimal places to include in the string representation. Default is 2.
+     * @param string $whole_str The string to use for the whole number part. Default is an empty string.
+     * @param string $decimal_str The string to use for the decimal part. Default is an empty string.
+     * @return string The string representation of the number.
      */
-    public function toString($number, $decimals = 2, $whole_str = '', $decimal_str = '')
+    public function toString(float|int $number, int $decimals = 2, string $whole_str = '', string $decimal_str = ''): string
     {
         return $this->toMoney($number, $decimals, $whole_str, $decimal_str);
     }
 
     /**
-     * Formatea y convierte un número a letras en formato facturación electrónica.
-     *
-     * @param int|float $number
-     * @param int       $decimals
-     * @param string    $currency
+     * Converts a number to its invoice representation in words.
      *
-     * @return string
+     * @param float $number The number to be converted.
+     * @param int $decimals The number of decimal places to consider. Default is 2.
+     * @param string $currency The currency to append to the converted number. Default is an empty string.
+     * @return string The number converted to its invoice representation in words, followed by the currency in uppercase.
      */
-    public function toInvoice($number, $decimals = 2, $currency = '')
+    public function toInvoice(float $number, int $decimals = 2, string $currency = ''): string
     {
         $this->checkApocope();
 
@@ -178,15 +198,17 @@ public function toInvoice($number, $decimals = 2, $currency = '')
             $splitNumber[1] = '00/100 ';
         }
 
-        return $this->glue($splitNumber) . mb_strtoupper($currency, 'UTF-8');
+        return $this->concat($splitNumber) . mb_strtoupper($currency, 'UTF-8');
     }
 
     /**
-     * Valida si debe aplicarse apócope de uno.
+     * Checks if the apocope flag is set to true and modifies the 'unidades' array accordingly.
+     *
+     * If the apocope flag is true, the value at index 1 of the 'unidades' array is set to 'UN '.
      *
      * @return void
      */
-    private function checkApocope()
+    private function checkApocope(): void
     {
         if ($this->apocope === true) {
             $this->unidades[1] = 'UN ';
@@ -194,13 +216,16 @@ private function checkApocope()
     }
 
     /**
-     * Formatea la parte entera del número a convertir.
+     * Converts a whole number to its corresponding word representation.
      *
-     * @param string $number
+     * This method takes a numeric string and converts it to its word representation.
+     * If the number is '0', it returns 'CERO '. Otherwise, it uses the convertNumber
+     * method to convert the number.
      *
-     * @return string
+     * @param string $number The numeric string to be converted.
+     * @return string The word representation of the number.
      */
-    private function wholeNumber($number)
+    private function wholeNumber(string $number): string
     {
         if ($number == '0') {
             $number = 'CERO ';
@@ -212,30 +237,38 @@ private function wholeNumber($number)
     }
 
     /**
-     * Concatena las partes formateadas del número convertido.
+     * Concatenates an array of strings with a connector.
      *
-     * @param array $splitNumber
+     * This method takes an array of strings, filters out any empty values,
+     * and then concatenates the remaining values using a connector string.
+     * The connector string is converted to uppercase using UTF-8 encoding.
      *
-     * @return string
+     * @param array $splitNumber The array of strings to concatenate.
+     * @return string The concatenated string.
      */
-    private function glue($splitNumber)
+    private function concat(array $splitNumber): string
     {
         return implode(' ' . mb_strtoupper($this->conector, 'UTF-8') . ' ', array_filter($splitNumber));
     }
 
     /**
-     * Convierte número a letras.
+     * Converts a numeric value into its corresponding Spanish words representation.
      *
-     * @param string $number
+     * This function handles numbers from 0 to 999,999,999. It divides the number into
+     * millions, thousands, and hundreds, and converts each group into words.
      *
-     * @return string
+     * @param int $number The number to be converted. Must be between 0 and 999,999,999.
+     * 
+     * @return string The number converted into Spanish words.
+     * 
+     * @throws ParseError If the number is less than 0 or greater than 999,999,999.
      */
-    private function convertNumber($number)
+    private function convertNumber(int $number): string
     {
         $converted = '';
 
-        if (($number < 0) || ($number > 999999999)) {
-            throw new ParseError('Wrong parameter number');
+        if (($number < 0) || !is_numeric($number)) {
+            throw new ParseError('Invalid number');
         }
 
         $numberStrFill = str_pad($number, 9, '0', STR_PAD_LEFT);
@@ -271,29 +304,35 @@ private function convertNumber($number)
     }
 
     /**
-     * @param string $n
+     * Converts a group of numbers into its corresponding Spanish words representation.
+     *
+     * This function converts a group of numbers (from 0 to 999) into its corresponding
+     * Spanish words representation. It handles the hundreds, tens, and units of the group.
      *
-     * @return string
+     * @param string $n The group of numbers to be converted.
+     * 
+     * @return string The group of numbers converted into Spanish words.
      */
-    private function convertGroup($n)
+
+    private function convertGroup(string $group): string
     {
         $output = '';
 
-        if ($n == '100') {
+        if ($group == '100') {
             $output = 'CIEN ';
-        } elseif ($n[0] !== '0') {
-            $output = $this->centenas[$n[0] - 1];
+        } elseif ($group[0] !== '0') {
+            $output = $this->centenas[$group[0] - 1];
         }
 
-        $k = intval(substr($n, 1));
+        $k = intval(substr($group, 1));
 
         if ($k <= 20) {
             $unidades = $this->unidades[$k];
         } else {
-            if (($k > 30) && ($n[2] !== '0')) {
-                $unidades = sprintf('%sY %s', $this->decenas[intval($n[1]) - 2], $this->unidades[intval($n[2])]);
+            if (($k > 30) && ($group[2] !== '0')) {
+                $unidades = sprintf('%sY %s', $this->decenas[intval($group[1]) - 2], $this->unidades[intval($group[2])]);
             } else {
-                $unidades = sprintf('%s%s', $this->decenas[intval($n[1]) - 2], $this->unidades[intval($n[2])]);
+                $unidades = sprintf('%s%s', $this->decenas[intval($group[1]) - 2], $this->unidades[intval($group[2])]);
             }
         }