Add legacy autoloader to support Matomo namespaces in 3.X (#16069)

LegacyAutoloader.php

@@ -0,0 +1,31 @@
+<?php
+
+class LegacyAutoloader
+{
+    public function __construct()
+    {
+        spl_autoload_register(array($this, 'load_class'));
+    }
+
+    public static function register()
+    {
+        new LegacyAutoloader();
+    }
+
+    public function load_class($className)
+    {
+        if (strpos($className, 'Matomo\\') === 0) {
+            $newName = 'Piwik' . substr($className, 6);
+            if (class_exists($newName) && !class_exists($className, false)) {
+                @class_alias($newName, $className);
+            }
+        } elseif (strpos($className, 'Piwik\\') === 0) {
+            $newName = 'Matomo' . substr($className, 5);
+            if (class_exists($newName) && !class_exists($className, false)) {
+                @class_alias($newName, $className);
+            }
+        }
+    }
+}
+
+LegacyAutoloader::register();

composer.json

@@ -144,7 +144,8 @@
             "HTML_": "libs/",
             "PEAR_": "libs/",
             "Archive_": "libs/"
-        }
+        },
+        "files": ["LegacyAutoloader.php"]
     },
     "autoload-dev": {
         "psr-4": {