Setting class alias only on php versions before 7

String is not a valid name in PHP 7 anymore.

config/bootstrap.php

@@ -18,7 +18,9 @@ use Cake\Routing\Router;
 define('TIME_START', microtime(true));
 
 // @deprecated Backward compatibility with 2.x series
-class_alias('Cake\Utility\Text', 'Cake\Utility\String');
+if (PHP_VERSION_ID < 70000) {
+    class_alias('Cake\Utility\Text', 'Cake\Utility\String');
+}
 
 require CAKE . 'basics.php';