Fix code quality issues.

composer.json

@@ -35,9 +35,12 @@
 		"overtrue/phplint": "^9.0",
 		"php-coveralls/php-coveralls": "^2.4",
 		"phpmd/phpmd": "^2.4",
+		"phpstan/phpstan": "^1.11",
 		"pronamic/pronamic-cli": "^1.1",
 		"pronamic/wp-coding-standards": "^1.3",
 		"roots/wordpress": "^6.0",
+		"szepeviktor/phpstan-wordpress": "^1.3",
+		"vimeo/psalm": "^5.25",
 		"vlucas/phpdotenv": "^2.0",
 		"wp-phpunit/wp-phpunit": "^6.1",
 		"yoast/phpunit-polyfills": "^1.0"

phpstan.neon.dist

@@ -1,9 +1,7 @@
 includes:
-	- vendor-bin/phpstan/vendor/szepeviktor/phpstan-wordpress/extension.neon
+	- vendor/szepeviktor/phpstan-wordpress/extension.neon
 parameters:
 	customRulesetUsed: false
 	level: max
-	bootstrapFiles:
-		- tests/phpstan/bootstrap.php
 	paths:
 		- src/

src/Parser.php

@@ -91,7 +91,7 @@ public function parse( $string ) {
 				$full = sprintf( '-%s', $full );
 
 				// Remove minus from end of half.
-				$half = \substr( (string) $half, 0, -1 );
+				$half = \substr( $half, 0, -1 );
 			}
 
 			$string = $full . '.' . $half;

src/TaxedMoney.php

@@ -38,10 +38,10 @@ class TaxedMoney extends Money {
 	/**
 	 * Construct and initialize money object.
 	 *
-	 * @param mixed           $value          Amount value.
-	 * @param Currency|string $currency       Currency.
-	 * @param mixed           $tax_value      Tax value.
-	 * @param mixed           $tax_percentage Tax percentage.
+	 * @param mixed                 $value          Amount value.
+	 * @param Currency|string       $currency       Currency.
+	 * @param float|int|string|null $tax_value      Tax value.
+	 * @param mixed                 $tax_percentage Tax percentage.
 	 */
 	public function __construct( $value = 0, $currency = 'EUR', $tax_value = null, $tax_percentage = null ) {
 		$value = Number::from_mixed( $value );
@@ -63,7 +63,7 @@ public function __construct( $value = 0, $currency = 'EUR', $tax_value = null, $
 			 */
 			$one_percent_value = $value->divide( $percentage );
 
-			$tax_value = $one_percent_value->multiply( $tax_percentage );
+			$tax_value = (string) $one_percent_value->multiply( $tax_percentage );
 		}
 
 		$this->set_tax_value( $tax_value );