Fix a bunch of deprecation warnings

By setting $_SERVER["PATH_INFO"] and $_REQUEST['page'] to empty
strings if they are not already defined. This is to avoid passing
`null` to `preg_match()` and `str_replace()` (used in
`sanitizeFilename()`), since this is deprecated in PHP 8.1.

This approach is recommended here:

https://php.watch/versions/8.1/internal-func-non-nullable-null-deprecation

index.php

@@ -214,6 +214,14 @@ function file_put_contents($n, $d)
 		}
     }
 }
+// Support PHP 8.1 by setting two predefined variables to empty strings if
+// not already defined. Fixes a bunch of deprecation warnings.
+
+if (!isset($_SERVER["PATH_INFO"]))
+  $_SERVER["PATH_INFO"] = '';
+if (!isset($_REQUEST['page']))
+  $_REQUEST['page'] = '';
+
 
 // Main code