PHP: How do I best silence a warning about unused variables?

I have an application in which i have to declare (override) methods inherited from an interface. However, these methods have parameters that are not used in my implementation.

class MyImplementation implements ISomething {

  public function foo($bar) {
    // code that does nothing with $bar
    ...
  }

  // other methods
  ...
}

How can I mark $bar as unused in the source code, in C++ this is possible.

In other words, how can I do THIS in PHP?