Query regarding dependency Injection in PHP

Question

I've been reading up on dependency injection and I think I pretty much understand the concepts, including constructor injection and setter injection.

However, I am unsure what I would do in the following scenario:

Say I have a class that has many methods, but only one of those methods requires a certain other object. How do I go about providing this object to the method? Should I not just directly pass the object to method as a parameter like so:

class aClass
{
    function aMethod(anotherClass $anotherClass)
    {
        anotherClass->hello();
    }

    function otherMethods()
    {
    }
    .....
}

The other option would be to provide it to the entire class, which seems unnecessary. like so:

class aClass
{
    protected $_anotherClass;

    function aMethod()
    {
        anotherClass->hello();
    }

    function otherMethods()
    {
    }
    .....

    function setAnotherClass(anotherClass $anotherClass)
    {
            $this->_anotherClass;
    }
}

When should I pass an object to a method and when should I use dependency injection via constructors/setters? Is there a rule of thumb or a best practice for when to use one of these two options?

Thanks for your help in advance.

Answer 1

You mentioned a rule of thumb, so here it is:

Ask yourself the question: could someone legitimately call this method twice on the same object, providing different values for the argument in the two calls?

If yes, then leave it as a parameter. If no, then it's a good candidate for injection as you describe.

This is a good example of a method of the first type, where removing the parameter and injecting a dependency in the class instead would be a really bad idea. C# instead of PHP, but you get the picture.