Add an interface for singleton requirement

src/Singleton/ISingleton.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace jrosset\Singleton;
+
+/**
+ * Interface for class that must implements a singleton
+ */
+interface ISingleton {
+    /**
+     * Get the instance
+     *
+     * If the instance is not already created then create it
+     *
+     * @return static The instance
+     */
+    public static function getInstance (): self;
+}

src/Singleton/TSingleton.php

@@ -3,7 +3,9 @@
 namespace jrosset\Singleton;
 
 /**
- * Trait for singleton design pattern
+ * Trait for simple singleton
+ *
+ * Initialization through the constructor
  */
 trait TSingleton {
     /**

tests/test.php

@@ -3,9 +3,10 @@
 
 require_once __DIR__ . '/../vendor/autoload.php';
 
+use jrosset\Singleton\ISingleton;
 use jrosset\Singleton\TSingleton;
 
-class Test {
+class Test implements ISingleton {
     use TSingleton;
 
     private int $cpt = 0;