Require PHP >= 8.1.0

Adds a check for the minimum required PHP version.
As enumerations are used, at least PHP 8.1 is required.
It's also the required version for PhD.

Author: MauricioFauth

configure.php

@@ -23,6 +23,10 @@
 ini_set( 'display_startup_errors' , 1 );
 error_reporting( E_ALL );
 
+if (PHP_VERSION_ID < 80100) {
+    throw new RuntimeException('PHP version ">= 8.1.0" is required. You are running ' . PHP_VERSION . '.');
+}
+
 ob_implicit_flush();
 libxml_use_internal_errors(true);