依赖注入。 这是正确的吗？

Question

Another question trying to verify the understanding of OOP dependencies of classes.

Although I knew PHP 4.* quite well, I have only recently restarted programming, and thus started working on OOP so please be gentle ;)

Classes should be as independent as possible I understand. So, when I have a class institute that gets information from a database, I would have to inject institute with an instance of the database class, right?:

$dbh = new database();
$a = new institute();

$a->SetDBI($dbh);

In database, I have a method pulling one record from the database, and is fed with the table table, the ID column and the actual ID of the record to be pulled.

Now, say I want one method in Institute that gets one institute. To me it would make sense to then use the database class method getone() :

public function GetInstitute()
{
    $record = $this->dbi->GetOneRecord('table', 'column', $this->id);
}

Is this the right way of going about it? It feel that I am still building dependencies between classes this way?

The question is: am I building dependencies that should not be here? Or is it standard practice to use methods from one class in another class, as shown in the example?

Answer 1

This is right. You still have some dependencies between classes, but only to a certain degree. Actually (because of PHP's weak typing), you can pass any class, and it will work as long as it implements the GetOneRecord method properly.

To make it a little more strict, you can use a base class or an interface and add type hinting to the setDBI method, but otherwise, this is the way to go.

But for the implementation detail, I'm not so sure. I see you pass a table name and column name to the database object. That is not right. You don't want your Institution to know how and where it is saved. I'd rather see that you passed an object implementing a LoadInstitution($id) method. But I find it hard to give you a solid example, because it's unclear to me what GetInstitute is meant to do in the first place.