class Database
{
    // Let's pretend we have boatloads of code here
}

// Now, the model class NEEDS database class in order to work, we can say
// that the model DEPENDS on the database class, since the class needs
// database instance to work, we INJECT the database object to the model
// class via constructor, this is called constructor injection IIRC

class BlogModel
{
    protected $db;

    // Inject a database instance via constructor
    public function __constructor(Database $db)
    {
        $this->db = $db;
    }
}

// Create an instance of database
$db = new Database('localhost', 'root', 'dkjfkf', 'db_name');

// Whenever we need the BlogModel, inject Database instance via the constructor
$blogmodel = new BlogModel($db);

// That's dependency injection in a nutshell