symfony - Why to use Dependency Injection components in PHP frameworks

Solution:

Why should you use Dependency Injection over the Singleton pattern?

Let's assume we have a Singleton object namedDatabaseConnection which wraps a connection to a MySQL database for us and does some other neat things, who knows. Because reusing code is a good thing, we use this object in a lot of projects.

What if at some point we decide to switch one of our projects from MySQL to another database product? We would have to modify every place where we call theDatabaseConnection object and replace it with our new implementation. Or, we could modify the class itself -- but we still want to use the original one with other projects, so we end up with two implementations with the same name which is just asking for trouble, really.

And what about unit tests? We do those, of course, because we are good developers! But if we unit test a function that uses the database, we don't want the test to actually rely on the database or even change things there. There's no way to replace theDatabaseConnection with a mock object (that just returns static data) because our project is tightly coupled to it.

That's what Dependency Injection does: It helps to prevent tight coupling. If we inject the connection with$someObject->setDatabaseConnection($databaseConnection), we can inject any object there that behaves like the original one. We can inject mock objects, alternative implementations or extensions that inherit the original class.

Now a Dependency Injection Container is just a nice helper to manage object instances and their dependencies more easily, but it's not needed for doing Dependency Injection.

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