diff --git a/.gitattributes b/.gitattributes
index 75cec1e..a2648e6 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -3,5 +3,6 @@
 /phpunit.xml.dist export-ignore
 /phpunit.7.5.xml export-ignore
 /tests/ export-ignore
+/phpstan.neon.dist export-ignore
 /.github/ export-ignore
 /Docs/ export-ignore
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 1293acd..2e28dc7 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -13,7 +13,7 @@ jobs:
             fail-fast: false
             matrix:
                 operating-system: [ubuntu-latest]
-                php-versions: ['7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3']
+                php-versions: ['7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3', '8.4']
         runs-on: ${{ matrix.operating-system }}
         steps:
             - name: Checkout
@@ -32,3 +32,8 @@ jobs:
                 else
                     vendor/bin/phpunit
                 fi
+            - name: Phpstan
+              run: |
+                if [ 1 -eq "$(echo "${{ matrix.php-versions }} > 7.1" | bc)" ]; then
+                    vendor/bin/phpstan
+                fi
diff --git a/.gitignore b/.gitignore
index 5b3201d..3f1fe1e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,4 @@ nbproject*
 /phpunit.xml
 /.phpunit.cache/
 .unlock_css_optimiser
+/phpstan.neon
diff --git a/class.csstidy.php b/class.csstidy.php
index d15c585..29a8d33 100644
--- a/class.csstidy.php
+++ b/class.csstidy.php
@@ -213,7 +213,7 @@ class csstidy {
 	 * @access private
 	 */
 	public $from = array();
-	/**
+
 	/**
 	 * =true if in invalid at-rule
 	 * @var bool
@@ -254,8 +254,10 @@ class csstidy {
 	/**
 	 * Various CSS Data for CSSTidy
 	 * @var array
+	 * 
+	 * @todo array{csstidy:array<string,mixed>}
 	 */
-	public $data = array();
+	public $data;
 
 	public $template;
 
@@ -266,6 +268,7 @@ class csstidy {
 	 */
 	public function __construct() {
 		$data = array();
+		// $data = ['csstidy' => []];
 		include(__DIR__ . DIRECTORY_SEPARATOR . 'data.inc.php');
 		$this->data = $data;
 
@@ -309,6 +312,7 @@ public function __construct() {
 		$this->set_cfg('template','default'); // call load_template
 		$this->optimise = new csstidy_optimise($this);
 
+		// @phpstan-ignore-next-line
 		$this->tokens_list = & $this->data['csstidy']['tokens'];
 	}
 
@@ -358,7 +362,7 @@ public function _load_template($template) {
 
 	/**
 	 * Set the value of a setting.
-	 * @param string $setting
+	 * @param array|string $setting
 	 * @param mixed $value
 	 * @access public
 	 * @return bool
@@ -575,6 +579,8 @@ public function is_token(&$string, $i) {
 	 */
 	public function parse($string) {
 		// Temporarily set locale to en_US in order to handle floats properly
+		// @todo Something $old = @setlocale(LC_ALL, '0');
+		// @see https://www.php.net/manual/en/function.setlocale.php#locales
 		$old = @setlocale(LC_ALL, 0);
 		@setlocale(LC_ALL, 'C');
 
@@ -1091,6 +1097,7 @@ public function css_add_property($media, $selector, $property, $new_val) {
 	 */
 	public function css_check_last_media_section_or_inc($media) {
 		// are we starting?
+		// @phpstan-ignore-next-line
 		if (!$this->css || !is_array($this->css) || empty($this->css)) {
 			return $media;
 		}
@@ -1119,7 +1126,7 @@ public function css_check_last_media_section_or_inc($media) {
 	 * to avoid merging
 	 *
 	 * @param string $current_media
-	 * @param string $media
+	 * @param string $new_media
 	 * @param bool $at_root
 	 * @return string
 	 */
@@ -1233,6 +1240,7 @@ public function css_new_property($media, $selector, $property) {
 	 */
 	public function merge_css_blocks($media, $selector, $css_add) {
 		foreach ($css_add as $property => $value) {
+			// @phpstan-ignore-next-line
 			$this->css_add_property($media, $selector, $property, $value, false);
 		}
 	}
@@ -1321,7 +1329,7 @@ public function property_is_valid($property) {
 	 * format("abc",'def') => format("abc","def")
 	 * format("abc, def, ghi") => format("abc, def, ghi")
 	 *
-	 * @param string
+	 * @param string $value
 	 * @return array
 	 */
 	public function parse_string_list($value) {
diff --git a/class.csstidy_optimise.php b/class.csstidy_optimise.php
index a94b7cd..1878e74 100644
--- a/class.csstidy_optimise.php
+++ b/class.csstidy_optimise.php
@@ -56,7 +56,7 @@ class csstidy_optimise {
 
 	/**
 	 * Constructor
-	 * @param array $css contains the class csstidy
+	 * @param csstidy $css contains the class csstidy
 	 * @access private
 	 * @version 1.0
 	 */
@@ -473,7 +473,7 @@ public function compress_numbers($subvalue) {
 	 * Checks if a given string is a CSS valid number. If it is,
 	 * an array containing the value and unit is returned
 	 * @param string $string
-	 * @return array ('unit' if unit is found or '' if no unit exists, number value) or false if no number
+	 * @return array|false ('unit' if unit is found or '' if no unit exists, number value) or false if no number
 	 */
 	public function AnalyseCssNumber($string) {
 		// most simple checks first
@@ -516,7 +516,7 @@ public function AnalyseCssNumber($string) {
 	 * Merges selectors with same properties. Example: a{color:red} b{color:red} -> a,b{color:red}
 	 * Very basic and has at least one bug. Hopefully there is a replacement soon.
 	 * @param array $array
-	 * @return array
+	 * @return void
 	 * @access public
 	 * @version 1.2
 	 */
@@ -637,7 +637,7 @@ public function dissolve_4value_shorthands($property, $value, $shorthands = null
 	 * @param string $value
 	 * @return array
 	 * @version 1.0
-	 * @use dissolve_4value_shorthands()
+	 * @uses dissolve_4value_shorthands()
 	 * @see merge_4value_radius_shorthands()
 	 */
 	public function dissolve_4value_radius_shorthands($property, $value) {
@@ -750,7 +750,7 @@ public function merge_4value_shorthands($array, $shorthands = null) {
 	 * @param array $array
 	 * @return array
 	 * @version 1.2
-	 * @use merge_4value_shorthands()
+	 * @uses merge_4value_shorthands()
 	 * @see dissolve_4value_radius_shorthands()
 	 */
 	public function merge_4value_radius_shorthands($array) {
@@ -1043,6 +1043,7 @@ public function merge_font($input_css) {
 		// Skip if not font-family and font-size set
 		if (isset($input_css['font-family']) && isset($input_css['font-size']) && $input_css['font-family'] != 'inherit') {
 			// fix several words in font-family - add quotes
+			// @phpstan-ignore-next-line
 			if (isset($input_css['font-family'])) {
 				$families = explode(',', $input_css['font-family']);
 				$result_families = array();
diff --git a/class.csstidy_print.php b/class.csstidy_print.php
index 2567342..077cf08 100644
--- a/class.csstidy_print.php
+++ b/class.csstidy_print.php
@@ -75,7 +75,7 @@ class csstidy_print {
 
 	/**
 	 * Constructor
-	 * @param array $css contains the class csstidy
+	 * @param csstidy $css contains the class csstidy
 	 * @access private
 	 * @version 1.0
 	 */
@@ -446,7 +446,7 @@ public function get_ratio() {
 	/**
 	 * Get difference between the old and new code in bytes and prints the code if necessary.
 	 * @access public
-	 * @return string
+	 * @return int|string
 	 * @version 1.1
 	 */
 	public function get_diff() {
diff --git a/composer.json b/composer.json
index 84e39f9..37d18f4 100644
--- a/composer.json
+++ b/composer.json
@@ -17,6 +17,7 @@
         "php": "^7.1 || ^8"
     },
     "require-dev": {
+        "phpstan/phpstan": "^1.4",
         "phpunit/phpunit": "^7.5 || ^8.5 || ^9.5 || ^10.5"
     },
     "autoload": {
diff --git a/phpstan-baseline.neon b/phpstan-baseline.neon
new file mode 100644
index 0000000..aab4991
--- /dev/null
+++ b/phpstan-baseline.neon
@@ -0,0 +1,2 @@
+parameters:
+	ignoreErrors: []
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
new file mode 100644
index 0000000..6e9c8df
--- /dev/null
+++ b/phpstan.neon.dist
@@ -0,0 +1,10 @@
+includes:
+  - phpstan-baseline.neon
+
+parameters:
+  phpVersion: 70100
+  paths:
+    - class.csstidy.php
+    - class.csstidy_optimise.php
+    - class.csstidy_print.php
+  level: 4