Some developers would say this is the most important principle to design loose coupled system. That's right. But you can't achieve it if you are not ISP and LSP in the first place.
This example is very common :
public function __construct()
{
$this->delegation = new Strategy();
}The birth of a new object is a critical moment. Don't do it casually. When you're typing "new", be sure it means your code cannot evolve.
About using a singleton or a static factory instead of "new" : it is a bad pratice too. You can't extend this code without breaking it and you can't mockup the delegation in unit tests. "Static" means "cannot evolve" by definition.
If you follow DIP, you would have :
public function __construct(StrategyInterface $strat)
{
$this->delegation = $strat;
}Notice that we rely on interfaces.
You should use design patterns like factory method, abstract factory and/or Dependency injection container like Pimple.
And if your class needs to create objects on demand, inject a factory.