diff --git a/.travis.yml b/.travis.yml
index 8435b1a..f402337 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,5 +2,8 @@ language: php
 php:
   - 7.3
   - 7.2
-before_script:
+install:
   - composer install --no-interaction --prefer-dist
+script:
+  - ./vendor/bin/phpunit
+  - ./vendor/bin/phpstan analyse
diff --git a/composer.json b/composer.json
index 833e60d..783edf4 100644
--- a/composer.json
+++ b/composer.json
@@ -10,7 +10,8 @@
 	"require": {
 		"php": ">=7.2.0",
 		"nikic/php-parser": "~4.2.0",
-		"latte/latte": "~2.5.0"
+		"latte/latte": "~2.5.0",
+		"nette/utils": "^3.0"
 	},
 	"autoload": {
 		"psr-4": {"Vodacek\\GettextExtractor\\" : "src"}
@@ -25,6 +26,8 @@
 		"gettext-extractor.php"
 	],
 	"require-dev": {
-		"phpunit/phpunit": "^8"
+		"phpunit/phpunit": "^8",
+		"phpstan/phpstan-shim": "^0.11.5",
+		"phpstan/phpstan-phpunit": "^0.11.0"
 	}
 }
diff --git a/gettext-extractor.php b/gettext-extractor.php
index efbf897..8731264 100755
--- a/gettext-extractor.php
+++ b/gettext-extractor.php
@@ -75,9 +75,9 @@
 		$keywords[] = array(
 			'filter' => $filter,
 			'function' => $function,
-			'singular' => isset($params[0]) ? $params[0] : null,
-			'plural' => isset($params[1]) ? $params[1] : null,
-			'context' => isset($params[2]) ? $params[2] : null
+			'singular' => isset($params[0]) ? (int) $params[0] : null,
+			'plural' => isset($params[1]) ? (int) $params[1] : null,
+			'context' => isset($params[2]) ? (int) $params[2] : null
 		);
 	}
 }
diff --git a/makefile b/makefile
new file mode 100644
index 0000000..5804cfd
--- /dev/null
+++ b/makefile
@@ -0,0 +1,7 @@
+check: test phpstan
+
+test:
+	./vendor/bin/phpunit
+
+phpstan:
+	./vendor/bin/phpstan analyse
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
new file mode 100644
index 0000000..062f63a
--- /dev/null
+++ b/phpstan.neon.dist
@@ -0,0 +1,13 @@
+parameters:
+	level: 7
+	paths:
+		- src
+		- tests
+#		- gettext-extractor.php
+	excludes_analyse:
+		- tests/integration/data
+		- tests/unit/data
+
+includes:
+	- vendor/phpstan/phpstan-phpunit/extension.neon
+	- vendor/phpstan/phpstan-phpunit/rules.neon
diff --git a/src/Extractor.php b/src/Extractor.php
index c27fc25..1924d81 100644
--- a/src/Extractor.php
+++ b/src/Extractor.php
@@ -8,6 +8,7 @@
 
 namespace Vodacek\GettextExtractor;
 
+use Nette\Utils\FileSystem;
 use RecursiveDirectoryIterator;
 use RecursiveIteratorIterator;
 use RuntimeException;
@@ -83,7 +84,7 @@ protected function throwException(string $message): void {
 	/**
 	 * Scans given files or directories and extracts gettext keys from the content
 	 *
-	 * @param string|array $resource
+	 * @param string|string[] $resource
 	 * @return self
 	 */
 	public function scan($resource): self {
@@ -237,7 +238,7 @@ public function setMeta(string $key, string $value): self {
 	 * @return self
 	 */
 	public function save(string $outputFile, array $data = null): self {
-		file_put_contents($outputFile, $this->formatData($data ?: $this->data));
+		FileSystem::write($outputFile, $this->formatData($data ?: $this->data));
 		return $this;
 	}
 
diff --git a/src/Filters/AFilter.php b/src/Filters/AFilter.php
index 33d081e..87a7c76 100644
--- a/src/Filters/AFilter.php
+++ b/src/Filters/AFilter.php
@@ -18,10 +18,10 @@ abstract class AFilter {
 	/**
 	 * Includes a function to parse gettext phrases from
 	 *
-	 * @param $functionName string
-	 * @param $singular int
-	 * @param $plural int|null
-	 * @param $context int|null
+	 * @param string $functionName
+	 * @param int $singular
+	 * @param int|null $plural
+	 * @param int|null $context
 	 * @return self
 	 */
 	public function addFunction(string $functionName, int $singular = 1, int $plural = null, int $context = null): self {
@@ -50,7 +50,7 @@ public function addFunction(string $functionName, int $singular = 1, int $plural
 	/**
 	 * Excludes a function from the function list
 	 *
-	 * @param $functionName
+	 * @param string $functionName
 	 * @return self
 	 */
 	public function removeFunction(string $functionName): self {
diff --git a/src/Filters/LatteFilter.php b/src/Filters/LatteFilter.php
index a61d076..1716cf4 100644
--- a/src/Filters/LatteFilter.php
+++ b/src/Filters/LatteFilter.php
@@ -8,6 +8,10 @@
 
 namespace Vodacek\GettextExtractor\Filters;
 
+use Nette\Utils\FileSystem;
+use PhpParser\Node\Expr\FuncCall;
+use PhpParser\Node\Scalar\String_;
+use PhpParser\Node\Stmt\Expression;
 use Vodacek\GettextExtractor\Extractor;
 use PhpParser;
 use Latte;
@@ -29,7 +33,7 @@ public function extract(string $file): array {
 		$data = array();
 
 		$latteParser = new Latte\Parser();
-		$tokens = $latteParser->parse(file_get_contents($file));
+		$tokens = $latteParser->parse(FileSystem::read($file));
 
 		$functions = array_keys($this->functions);
 		usort($functions, static function(string $a, string $b) {
@@ -49,25 +53,30 @@ public function extract(string $file): array {
 			$value = $this->trimMacroValue($name, $token->value);
 			$stmts = $phpParser->parse("<?php\nf($value);");
 
-			foreach ($this->functions[$name] as $definition) {
-				$message = $this->processFunction($definition, $stmts[0]->expr);
-				if ($message) {
-					$message[Extractor::LINE] = $token->line;
-					$data[] = $message;
+			if ($stmts === null) {
+				continue;
+			}
+			if ($stmts[0] instanceof Expression && $stmts[0]->expr instanceof FuncCall) {
+				foreach ($this->functions[$name] as $definition) {
+					$message = $this->processFunction($definition, $stmts[0]->expr);
+					if ($message) {
+						$message[Extractor::LINE] = $token->line;
+						$data[] = $message;
+					}
 				}
 			}
 		}
 		return $data;
 	}
 
-	private function processFunction(array $definition, PhpParser\Node\Expr\FuncCall $node): array {
+	private function processFunction(array $definition, FuncCall $node): array {
 		$message = [];
 		foreach ($definition as $type => $position) {
 			if (!isset($node->args[$position - 1])) {
 				return [];
 			}
 			$arg = $node->args[$position - 1]->value;
-			if ($arg instanceof PhpParser\Node\Scalar\String_) {
+			if ($arg instanceof String_) {
 				$message[$type] = $arg->value;
 			} else {
 				return [];
diff --git a/src/Filters/PHPFilter.php b/src/Filters/PHPFilter.php
index 31297ce..e155b0b 100644
--- a/src/Filters/PHPFilter.php
+++ b/src/Filters/PHPFilter.php
@@ -7,13 +7,23 @@
 
 namespace Vodacek\GettextExtractor\Filters;
 
+use Nette\Utils\FileSystem;
 use PhpParser;
+use PhpParser\Node;
+use PhpParser\Node\Arg;
+use PhpParser\Node\Expr\Array_;
+use PhpParser\Node\Expr\FuncCall;
+use PhpParser\Node\Expr\MethodCall;
+use PhpParser\Node\Expr\StaticCall;
+use PhpParser\Node\Identifier;
+use PhpParser\Node\Name;
+use PhpParser\Node\Scalar\String_;
 use Vodacek\GettextExtractor\Extractor;
 
 class PHPFilter extends AFilter implements IFilter, PhpParser\NodeVisitor {
 
 	/** @var array */
-	private $data;
+	private $data = [];
 
 	public function __construct() {
 		$this->addFunction('gettext', 1);
@@ -29,47 +39,58 @@ public function __construct() {
 	public function extract(string $file): array {
 		$this->data = array();
 		$parser = (new PhpParser\ParserFactory())->create(PhpParser\ParserFactory::PREFER_PHP7);
-		$stmts = $parser->parse(file_get_contents($file));
+		$stmts = $parser->parse(FileSystem::read($file));
+		if ($stmts === null) {
+			return [];
+		}
 		$traverser = new PhpParser\NodeTraverser();
 		$traverser->addVisitor($this);
 		$traverser->traverse($stmts);
 		$data = $this->data;
-		$this->data = null;
+		$this->data = [];
 		return $data;
 	}
 
-	public function enterNode(PhpParser\Node $node) {
+	public function enterNode(Node $node) {
 		$name = null;
-		if (($node instanceof PhpParser\Node\Expr\MethodCall || $node instanceof PhpParser\Node\Expr\StaticCall) && $node->name instanceof PhpParser\Node\Identifier && is_string($node->name->name)) {
+		$args = [];
+		if (($node instanceof MethodCall || $node instanceof StaticCall) && $node->name instanceof Identifier && is_string($node->name->name)) {
 			$name = $node->name->name;
-		} elseif ($node instanceof PhpParser\Node\Expr\FuncCall && $node->name instanceof PhpParser\Node\Name) {
+			$args = $node->args;
+		} elseif ($node instanceof FuncCall && $node->name instanceof Name) {
 			$parts = $node->name->parts;
 			$name = array_pop($parts);
+			$args = $node->args;
 		} else {
-			return;
+			return null;
 		}
 		if (!isset($this->functions[$name])) {
-			return;
+			return null;
 		}
 		foreach ($this->functions[$name] as $definition) {
-			$this->processFunction($definition, $node);
+			$this->processFunction($definition, $node, $args);
 		}
 	}
 
-	private function processFunction(array $definition, PhpParser\Node $node) {
+	/**
+	 * @param array $definition
+	 * @param Node $node
+	 * @param Arg[] $args
+	 */
+	private function processFunction(array $definition, Node $node, array $args) {
 		$message = array(
 			Extractor::LINE => $node->getLine()
 		);
 		foreach ($definition as $type => $position) {
-			if (!isset($node->args[$position - 1])) {
+			if (!isset($args[$position - 1])) {
 				return;
 			}
-			$arg = $node->args[$position - 1]->value;
-			if ($arg instanceof PhpParser\Node\Scalar\String_) {
+			$arg = $args[$position - 1]->value;
+			if ($arg instanceof String_) {
 				$message[$type] = $arg->value;
-			} elseif ($arg instanceof PhpParser\Node\Expr\Array_) {
+			} elseif ($arg instanceof Array_) {
 				foreach ($arg->items as $item) {
-					if ($item->value instanceof PhpParser\Node\Scalar\String_) {
+					if ($item->value instanceof String_) {
 						$message[$type][] = $item->value->value;
 					}
 				}
@@ -99,6 +120,6 @@ public function afterTraverse(array $nodes) {
 	public function beforeTraverse(array $nodes) {
 	}
 
-	public function leaveNode(PhpParser\Node $node) {
+	public function leaveNode(Node $node) {
 	}
 }
diff --git a/src/NetteExtractor.php b/src/NetteExtractor.php
index f25b65a..3a8caff 100644
--- a/src/NetteExtractor.php
+++ b/src/NetteExtractor.php
@@ -8,12 +8,12 @@
 
 namespace Vodacek\GettextExtractor;
 
+use Vodacek\GettextExtractor\Filters\LatteFilter;
+use Vodacek\GettextExtractor\Filters\PHPFilter;
+
 class NetteExtractor extends Extractor {
 
-	/**
-	 * @param string|bool $logToFile
-	 */
-	public function __construct($logToFile = false) {
+	public function __construct(string $logToFile = 'php://stderr') {
 		parent::__construct($logToFile);
 
 		// Clean up...
@@ -28,11 +28,15 @@ public function __construct($logToFile = false) {
 
 		$this->addFilter('Latte', new Filters\LatteFilter());
 
-		$this->getFilter('PHP')
-				->addFunction('translate');
+		$phpFilter = $this->getFilter('PHP');
+		assert($phpFilter instanceof PHPFilter);
+
+		$phpFilter->addFunction('translate');
 
-		$this->getFilter('Latte')
-				->addFunction('!_')
+		$latteFilter = $this->getFilter('Latte');
+		assert($latteFilter instanceof LatteFilter);
+
+		$latteFilter->addFunction('!_')
 				->addFunction('_');
 	}
 
@@ -43,6 +47,8 @@ public function __construct($logToFile = false) {
 	 */
 	public function setupForms(): self {
 		$php = $this->getFilter('PHP');
+		assert($php instanceof PHPFilter);
+
 		$php->addFunction('setText')
 				->addFunction('setEmptyValue')
 				->addFunction('setValue')
@@ -80,6 +86,8 @@ public function setupForms(): self {
 	 */
 	public function setupDataGrid(): self {
 		$php = $this->getFilter('PHP');
+		assert($php instanceof PHPFilter);
+
 		$php->addFunction('addColumn', 2)
 				->addFunction('addNumericColumn', 2)
 				->addFunction('addDateColumn', 2)
diff --git a/tests/integration/NetteFormsTest.php b/tests/integration/NetteFormsTest.php
index 3ab723c..bf4a583 100644
--- a/tests/integration/NetteFormsTest.php
+++ b/tests/integration/NetteFormsTest.php
@@ -20,7 +20,11 @@ public function testForm(): void {
 		$this->object->setupForms();
 		$this->object->scan('tests/integration/data/form.php');
 		$temp = tempnam(sys_get_temp_dir(), __CLASS__);
+		if ($temp === false) {
+			throw new \RuntimeException('Failed to create temporary file.');
+		}
 		$this->object->save($temp);
 		$this->assertFileEquals(__DIR__.'/data/form.pot', $temp);
+		unlink($temp);
 	}
 }
diff --git a/tests/unit/GettextExtractor/Filters/AFilterTest.php b/tests/unit/GettextExtractor/Filters/AFilterTest.php
index 6c07f3a..f46abfe 100644
--- a/tests/unit/GettextExtractor/Filters/AFilterTest.php
+++ b/tests/unit/GettextExtractor/Filters/AFilterTest.php
@@ -1,15 +1,16 @@
 <?php
 declare(strict_types=1);
 
-namespace Vodacek\GettextExtractor\Tests\Integration;
+namespace Vodacek\GettextExtractor\Tests\Unit\GettextExtractor\Filters;
 
 use InvalidArgumentException;
+use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
 use Vodacek\GettextExtractor as GE;
 
 class AFilterTest extends TestCase {
 
-	/** @var GE\Filters\AFilter */
+	/** @var MockObject&GE\Filters\AFilter */
 	protected $object;
 
 	protected function setUp(): void {
diff --git a/tests/unit/GettextExtractor/Filters/FilterTest.php b/tests/unit/GettextExtractor/Filters/FilterTest.php
deleted file mode 100644
index 6dae945..0000000
--- a/tests/unit/GettextExtractor/Filters/FilterTest.php
+++ /dev/null
@@ -1,46 +0,0 @@
-<?php
-declare(strict_types=1);
-
-namespace Vodacek\GettextExtractor\Tests\Integration;
-
-use PHPUnit\Framework\TestCase;
-use Vodacek\GettextExtractor as GE;
-
-abstract class FilterTest extends TestCase {
-
-	/** @var GE\Filters\IFilter */
-	protected $object;
-
-	/** @var string */
-	protected $file;
-
-	public function testExtract(): void {
-		$messages = $this->object->extract($this->file);
-
-		$this->assertIsArray($messages);
-
-		$this->assertContains(array(
-			GE\Extractor::LINE => 2,
-			GE\Extractor::SINGULAR => 'A message!'
-		), $messages);
-
-		$this->assertContains(array(
-			GE\Extractor::LINE => 3,
-			GE\Extractor::SINGULAR => 'Another message!',
-			GE\Extractor::CONTEXT => 'context'
-		), $messages);
-
-		$this->assertContains(array(
-			GE\Extractor::LINE => 4,
-			GE\Extractor::SINGULAR => 'I see %d little indian!',
-			GE\Extractor::PLURAL => 'I see %d little indians!'
-		), $messages);
-
-		$this->assertContains(array(
-			GE\Extractor::LINE => 5,
-			GE\Extractor::SINGULAR => 'I see %d little indian!',
-			GE\Extractor::PLURAL => 'I see %d little indians!',
-			GE\Extractor::CONTEXT => 'context'
-		), $messages);
-	}
-}
diff --git a/tests/unit/GettextExtractor/Filters/LatteFilterTest.php b/tests/unit/GettextExtractor/Filters/LatteFilterTest.php
index 6e0e2cc..ca57704 100644
--- a/tests/unit/GettextExtractor/Filters/LatteFilterTest.php
+++ b/tests/unit/GettextExtractor/Filters/LatteFilterTest.php
@@ -1,15 +1,48 @@
 <?php
 declare(strict_types=1);
 
-namespace Vodacek\GettextExtractor\Tests\Integration;
+namespace Vodacek\GettextExtractor\Tests\Unit\GettextExtractor\Filters;
 
+use PHPUnit\Framework\TestCase;
 use Vodacek\GettextExtractor as GE;
 
-class LatteFilterTest extends FilterTest {
+class LatteFilterTest extends TestCase {
+
+	/** @var GE\Filters\LatteFilter */
+	private $object;
 
 	protected function setUp(): void {
 		$this->object = new GE\Filters\LatteFilter();
-		$this->file = __DIR__ . '/../../data/latte/default.latte';
+	}
+
+	public function testExtract(): void {
+		$messages = $this->object->extract(__DIR__ . '/../../data/latte/default.latte');
+
+		$this->assertIsArray($messages);
+
+		$this->assertContains(array(
+			GE\Extractor::LINE => 2,
+			GE\Extractor::SINGULAR => 'A message!'
+		), $messages);
+
+		$this->assertContains(array(
+			GE\Extractor::LINE => 3,
+			GE\Extractor::SINGULAR => 'Another message!',
+			GE\Extractor::CONTEXT => 'context'
+		), $messages);
+
+		$this->assertContains(array(
+			GE\Extractor::LINE => 4,
+			GE\Extractor::SINGULAR => 'I see %d little indian!',
+			GE\Extractor::PLURAL => 'I see %d little indians!'
+		), $messages);
+
+		$this->assertContains(array(
+			GE\Extractor::LINE => 5,
+			GE\Extractor::SINGULAR => 'I see %d little indian!',
+			GE\Extractor::PLURAL => 'I see %d little indians!',
+			GE\Extractor::CONTEXT => 'context'
+		), $messages);
 	}
 
 	public function testNoValidMessages(): void {
diff --git a/tests/unit/GettextExtractor/Filters/PHPFilterTest.php b/tests/unit/GettextExtractor/Filters/PHPFilterTest.php
index bbc2e45..2fda3b2 100644
--- a/tests/unit/GettextExtractor/Filters/PHPFilterTest.php
+++ b/tests/unit/GettextExtractor/Filters/PHPFilterTest.php
@@ -1,18 +1,49 @@
 <?php
 declare(strict_types=1);
 
-namespace Vodacek\GettextExtractor\Tests\Integration;
+namespace Vodacek\GettextExtractor\Tests\Unit\GettextExtractor\Filters;
 
+use PHPUnit\Framework\TestCase;
 use Vodacek\GettextExtractor as GE;
 
-require_once __DIR__ . '/FilterTest.php';
+class PHPFilterTest extends TestCase {
 
-class PHPFilterTest extends FilterTest {
+	/** @var GE\Filters\PHPFilter */
+	private $object;
 
 	protected function setUp(): void {
 		$this->object = new GE\Filters\PHPFilter();
 		$this->object->addFunction('addRule', 2);
-		$this->file = __DIR__ . '/../../data/php/default.php';
+	}
+
+	public function testExtract(): void {
+		$messages = $this->object->extract(__DIR__ . '/../../data/php/default.php');
+
+		$this->assertIsArray($messages);
+
+		$this->assertContains(array(
+			GE\Extractor::LINE => 2,
+			GE\Extractor::SINGULAR => 'A message!'
+		), $messages);
+
+		$this->assertContains(array(
+			GE\Extractor::LINE => 3,
+			GE\Extractor::SINGULAR => 'Another message!',
+			GE\Extractor::CONTEXT => 'context'
+		), $messages);
+
+		$this->assertContains(array(
+			GE\Extractor::LINE => 4,
+			GE\Extractor::SINGULAR => 'I see %d little indian!',
+			GE\Extractor::PLURAL => 'I see %d little indians!'
+		), $messages);
+
+		$this->assertContains(array(
+			GE\Extractor::LINE => 5,
+			GE\Extractor::SINGULAR => 'I see %d little indian!',
+			GE\Extractor::PLURAL => 'I see %d little indians!',
+			GE\Extractor::CONTEXT => 'context'
+		), $messages);
 	}
 
 	public function testNoValidMessages(): void {