I tried putting it in different spots in the code but I keep getting errors. The first error, when put at the end:

Warning: Undefined property: Opencart\System\Engine\Autoloader::$customer in testing2/catalog/controller/common/header.php on line 98Call to a member function isLogged() on null: in testing2/catalog/controller/common/header.php on line 98

I get the same error if I put it inside the function, and the second error if put at the top:

syntax error, unexpected token "if", expecting "function" or "const": in testing2/catalog/controller/common/header.php on line 13