diff --git a/src/Filter.php b/src/Filter.php
index 1a1f4e9..6dd9fa5 100644
--- a/src/Filter.php
+++ b/src/Filter.php
@@ -2,7 +2,7 @@
 
 namespace Pd\Version;
 
-final class Filter
+final class Filter implements IFilter
 {
 
 	public const CACHE_TAG = 'pd-version';
diff --git a/src/IFilter.php b/src/IFilter.php
new file mode 100644
index 0000000..e4144f8
--- /dev/null
+++ b/src/IFilter.php
@@ -0,0 +1,14 @@
+<?php declare(strict_types = 1);
+
+namespace Pd\Version;
+
+interface IFilter
+{
+
+	public function __invoke(
+		$url,
+		$directory = NULL,
+		$parameter = NULL
+	);
+
+}