diff --git a/README.md b/README.md
index 39cb8eb..9c894e0 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,8 @@
 # php-globalsnapshot
 
-Capture the current state of all [PHP superglobal variables](https://www.php.net/manual/en/language.variables.superglobals.php); which can then be restored to that state at a later time.
+Capture the current state of all [PHP superglobal variables](https://www.php.net/manual/en/language.variables.superglobals.php) - which can then be restored to the state of capture at a later time.
+
+**Supports PHP 8.1+**
 
 Example use:
 ```php
diff --git a/src/GlobalSnapshot.php b/src/GlobalSnapshot.php
index 3e9d2a2..e30d894 100644
--- a/src/GlobalSnapshot.php
+++ b/src/GlobalSnapshot.php
@@ -7,14 +7,14 @@
     // can be restored to this point in time by calling $this->restore();
     class GlobalSnapshot {
         // Declare all PHP superglobals
-        private array $_ENV;
-        private array $_GET;
-        private array $_POST;
-        private array $_FILES;
-        private array $_SERVER;
-        private array $_COOKIE;
-        private array $_REQUEST;
-        private array $_SESSION;
+        private readonly array $_ENV;
+        private readonly array $_GET;
+        private readonly array $_POST;
+        private readonly array $_FILES;
+        private readonly array $_SERVER;
+        private readonly array $_COOKIE;
+        private readonly array $_REQUEST;
+        private readonly array $_SESSION;
 
 		// Declare additional PHP globals (for PHP 8.0+ compatability)
 		// These will not be captured, or altered by GlobalSnapshot