diff --git a/composer.json b/composer.json
index 92e59f66..61a944cd 100644
--- a/composer.json
+++ b/composer.json
@@ -19,7 +19,8 @@
     },
     "require": {
         "php": "^7.2|^8.0",
-        "gettext/languages": "^2.3"
+        "gettext/languages": "^2.3",
+        "ext-json": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "^8.0|^9.0",
diff --git a/phpstan.dist.neon b/phpstan.dist.neon
index d2554326..f226e5e2 100644
--- a/phpstan.dist.neon
+++ b/phpstan.dist.neon
@@ -1,5 +1,6 @@
 parameters:
-    level: 5
+#    level: 5
+    level: 6
     paths:
     - src
     - tests
diff --git a/src/Comments.php b/src/Comments.php
index ca44c97e..03525f74 100644
--- a/src/Comments.php
+++ b/src/Comments.php
@@ -13,11 +13,21 @@
  * Class to manage the comments of a translation.
  *
  * @phpstan-consistent-constructor
+ *
+ * @phpstan-type CommentsType array<int, string>
+ *
+ * @implements IteratorAggregate<int, string>
  */
 class Comments implements JsonSerializable, Countable, IteratorAggregate
 {
+    /**
+     * @var CommentsType
+     */
     protected $comments = [];
 
+    /**
+     * @param array{comments: CommentsType} $state
+     */
     public static function __set_state(array $state): Comments
     {
         return new static(...$state['comments']);
@@ -51,9 +61,7 @@ public function delete(string ...$comments): self
         foreach ($comments as $comment) {
             $key = array_search($comment, $this->comments);
 
-            if (is_int($key)) {
-                array_splice($this->comments, $key, 1);
-            }
+            array_splice($this->comments, $key, 1);
         }
 
         return $this;
@@ -76,6 +84,9 @@ public function count(): int
         return count($this->comments);
     }
 
+    /**
+     * @return CommentsType
+     */
     public function toArray(): array
     {
         return $this->comments;
diff --git a/src/Flags.php b/src/Flags.php
index e8826bb8..a78079ca 100644
--- a/src/Flags.php
+++ b/src/Flags.php
@@ -13,11 +13,21 @@
  * Class to manage the flags of a translation.
  *
  * @phpstan-consistent-constructor
+ *
+ * @phpstan-type FlagsType array<int, string>
+ *
+ * @implements IteratorAggregate<int, string>
  */
 class Flags implements JsonSerializable, Countable, IteratorAggregate
 {
+    /**
+     * @var FlagsType
+     */
     protected $flags = [];
 
+    /**
+     * @param array{flags: FlagsType} $state
+     */
     public static function __set_state(array $state): Flags
     {
         return new static(...$state['flags']);
@@ -83,6 +93,9 @@ public function count(): int
         return count($this->flags);
     }
 
+    /**
+     * @return FlagsType
+     */
     public function toArray(): array
     {
         return $this->flags;
diff --git a/src/Generator/MoGenerator.php b/src/Generator/MoGenerator.php
index e8151016..59444733 100644
--- a/src/Generator/MoGenerator.php
+++ b/src/Generator/MoGenerator.php
@@ -8,6 +8,9 @@
 
 final class MoGenerator extends Generator
 {
+    /**
+     * @var bool
+     */
     private $includeHeaders = false;
 
     public function includeHeaders(bool $includeHeaders = true): self
diff --git a/src/Generator/PoGenerator.php b/src/Generator/PoGenerator.php
index 000c1a6a..9cb849f4 100644
--- a/src/Generator/PoGenerator.php
+++ b/src/Generator/PoGenerator.php
@@ -102,6 +102,8 @@ public function generateString(Translations $translations): string
 
     /**
      * Add one or more lines depending whether the string is multiline or not.
+     *
+     * @param array<int, string> $lines
      */
     private static function appendLines(array &$lines, string $prefix, string $name, string $value): void
     {
diff --git a/src/Headers.php b/src/Headers.php
index b6ad091f..0e3125d7 100644
--- a/src/Headers.php
+++ b/src/Headers.php
@@ -14,6 +14,10 @@
  * Class to manage the headers of translations.
  *
  * @phpstan-consistent-constructor
+ *
+ * @phpstan-type HeadersType array<string, string>
+ *
+ * @implements IteratorAggregate<string, string>
  */
 class Headers implements JsonSerializable, Countable, IteratorAggregate
 {
@@ -21,13 +25,22 @@ class Headers implements JsonSerializable, Countable, IteratorAggregate
     public const HEADER_PLURAL = 'Plural-Forms';
     public const HEADER_DOMAIN = 'X-Domain';
 
+    /**
+     * @var HeadersType
+     */
     protected $headers = [];
 
+    /**
+     * @param array{headers: HeadersType} $state
+     */
     public static function __set_state(array $state): Headers
     {
         return new static($state['headers']);
     }
 
+    /**
+     * @param HeadersType $headers
+     */
     public function __construct(array $headers = [])
     {
         $this->headers = $headers;
@@ -115,7 +128,7 @@ public function setPluralForm(int $count, string $rule): self
     /**
      * Returns the parsed plural definition.
      *
-     * @return array|null [count, rule]
+     * @return array{int, string}|null [count, rule]
      */
     public function getPluralForm(): ?array
     {
@@ -130,6 +143,9 @@ public function getPluralForm(): ?array
         return null;
     }
 
+    /**
+     * @return HeadersType
+     */
     public function toArray(): array
     {
         return $this->headers;
diff --git a/src/Loader/MoLoader.php b/src/Loader/MoLoader.php
index 5fc9419e..903ad737 100644
--- a/src/Loader/MoLoader.php
+++ b/src/Loader/MoLoader.php
@@ -11,8 +11,19 @@
  */
 final class MoLoader extends Loader
 {
+    /**
+     * @var string
+     */
     private $string;
+
+    /**
+     * @var int<0, max>
+     */
     private $position;
+
+    /**
+     * @var int<0, max>
+     */
     private $length;
 
     private const MAGIC1 = -1794895138;
@@ -132,6 +143,12 @@ private function readInt(string $byteOrder): int
         return (int) array_shift($read);
     }
 
+    /**
+     * @param string $byteOrder
+     * @param int    $count
+     *
+     * @return array<int, int>
+     */
     private function readIntArray(string $byteOrder, int $count): array
     {
         return unpack($byteOrder.$count, $this->read(4 * $count)) ?: [];
diff --git a/src/Loader/PoLoader.php b/src/Loader/PoLoader.php
index ca7c281e..92bcaa1b 100644
--- a/src/Loader/PoLoader.php
+++ b/src/Loader/PoLoader.php
@@ -153,6 +153,11 @@ public function loadString(string $string, ?Translations $translations = null):
         return $translations;
     }
 
+    /**
+     * @param string|null $string
+     *
+     * @return array<string, string>
+     */
     private static function parseHeaders(?string $string): array
     {
         if (empty($string)) {
diff --git a/src/Loader/StrictPoLoader.php b/src/Loader/StrictPoLoader.php
index fe66c86d..ab81b88f 100644
--- a/src/Loader/StrictPoLoader.php
+++ b/src/Loader/StrictPoLoader.php
@@ -437,6 +437,8 @@ private function processHeader(): void
 
     /**
      * Parses the translation header data into an array
+     *
+     * @return array<string, string>
      */
     private function readHeaders(string $data): array
     {
diff --git a/src/References.php b/src/References.php
index 274219fe..3ec70803 100644
--- a/src/References.php
+++ b/src/References.php
@@ -13,11 +13,21 @@
  * Class to manage the references of a translation.
  *
  * @phpstan-consistent-constructor
+ *
+ * @phpstan-type ReferencesType array<string, list<int>>
+ *
+ * @implements IteratorAggregate<string, list<int>>
  */
 class References implements JsonSerializable, Countable, IteratorAggregate
 {
+    /**
+     * @var ReferencesType
+     */
     protected $references = [];
 
+    /**
+     * @param array{references: ReferencesType} $state
+     */
     public static function __set_state(array $state): References
     {
         $references = new static();
@@ -67,6 +77,9 @@ public function count(): int
         }, 0);
     }
 
+    /**
+     * @return ReferencesType
+     */
     public function toArray(): array
     {
         return $this->references;