Declare Global Variable In OOP PHP

The $current_screen is passed as a variable to callables hooked from the current_screen hook.

Before this hook, the $current_screen isn’t set up, so globalizing the variable won’t do anything anyway.

Also, WordPress offers the convenience function get_current_screen() that returns the global $current_screen variable if it exists.

class wpse {
  protected $current_screen;
  public function load() {
    add_action( 'current_screen', [ $this, 'current_screen' ] );
  }
  public function current_screen( $current_screen ) {
    $this->current_screen = $current_screen;
  }
}
add_action( 'plugins_loaded', [ new wpse(), 'load' ] );