EmailNotifier.php

<?php

class EmailNotifier implements NotifierInterface {
    public function send()
    {
        // send email.
    }
}

NotifierInterface.php

<?php 

interface NotifierInterface {
    public function send();
}

using_ioc.php

<?php 

class User {
    protected $notifier;
    public function __construct(NotifierInterface $notifier)
    {
        $this->notifier = $notifier;
    }
    
    public function update()
    {
        // update user.
        $this->notifier->send();
    }
}

App::bind('NotifierInterface', 'EmailNotifier');

$user = App::make('User'); /* you can't use new User */

using_servicelocator.php

<?php 

class User {
    protected $notifier;
    public function __construct()
    {
        $this->notifier = App::make('NotifierInterface');
    }
    
    public function update()
    {
        // update user.
        $this->notifier->send();
    }
}

App::bind('NotifierInterface', 'EmailNotifier');

$user = App::make('User'); /* or new User */