Modular Programming with PHP 7

Dependency inversion principle

The dependency inversion principle states that entities should depend on abstractions and not on concretions. That is, a high level module should not depend on a low level module, rather the abstraction. As per the definition found on Wikipedia:

"One should depend upon abstractions. Do not depend upon concretions."

This principle is important as it plays a major role in decoupling our software.

The following is an example of a class that violates the DIP:

class Mailer {
    // Implementation...
}

class NotifySubscriber {
    public function notify($emailTo) {
        $mailer = new Mailer();
        $mailer->send('Thank you for...', $emailTo);
    }
}

Here we can see a notify method within the NotifySubscriber class coding in a dependency towards ...